How AR uses aether consumption to manage tiers (and which tier you're likely to sit in)

We get stuck in a tier when we are unable to gain enough lift in one to promote into the next tier after lift lost on defense gets subtracted.

I'm going to show how I reached my conclusions for those interested. Feel free to skip to the summary at the bottom.

Lift loss on defense is relatively easy to calculate at a very general level. There are 160 hours in a season, and it is only possible to lose lift every 20 hours, so with there being small gaps between those 20h periods, it is impossible to lose more than 7 times on defense. To the best of our knowledge at this point, 80 is the maximum lift loss. Maybe it goes up to 100 if you have 6 members. But for now we assume 7 x 80 = 560, and you get 20 back for every raider you down.

Offense we can calculate as number of raids multiplied by lift gained per raid, and the number of raids depends on your lift and the level on your aether structures. We get 1 full recharge at the beginning of the season and 6 daily recharges in the middle. Aether regained from breaking opponents' structures I will just subtract from aether per raid. If you have aether amphorae lv 2 and fountain lv 1, you get 450 aether per season; if your amphorae is lv 3 then you get 500 per season. I will use lv 2 for tiers 16-17 and lv 3 for tiers 18+ because that's where we get to the point where the amount of aether you need for one battle + 50 on the daily reset + the amount gained from breaking an opponent's structures during the free raid can exceed 150, which would result in your amphorae overflowing after the daily free raid.

T16: 6000-6499 lift
Minimum aether per raid: 69 - 14 = 55
Maximum aether per raid: 73 ( - 15 = 58)
450 aether from automatic regeneration + (7 free battles x 14 aether from breaking structures) = 520 aether
At 57 aether a raid, that can probably support 9 raids.
9 battles in addition to the free daily battle for 16 raids = maximum score gained on offense of 1600 lift
With perfect performance and losing no more than 400 on defense, promote to T18
With complete disaster on defense and winning at least 1060 lift on offense (missed 540 lift, so 27 units lost on 16 raids which is one unit per raid and a second one every other raid), promote to T17.
I would expect that a majority of players (especially those who read this) will surpass T16.

T17: 6500-7199 lift
Minimum aether per raid: 74 - 15 = 59
Maximum aether per raid: 80 ( - 16 = 64)
450 from automatic regenerations + (7 daily battles x 15 from breaking opponent aether structures) = 555 aether
It's marginal. That might support 8 matches, maybe 9, also depends on how much you lose on defense and how early in the week.
8 raids + 7 free = max 1500 gain on offense.
If defense loses completely (-560), offense must also miss 260 lift in units lost, which is 13 units, meaning one per raid excepting two deathless raids.
Unless fountains become upgradable, this is where some players will start to get stuck. But I think a lot of players will move beyond.

T18: 7200-8199 lift
Minimum aether per raid: 81 - 16 = 65
Maximum aether per raid: 90 (- 18 = 72)
500 aether from automatic regeneration + (7 free battles x 16 aether regenerated) = 582
At 70 aether a match, that will support 8 raids.
8 raids + 7 free = maximum 1500 lift on offense
On a perfect performance from offense, defense must lose 520+ (i.e. fails to get 3 kills all week) for the player to miss T19.
If defense loses completely (-560) and offense fails to gain 560 lift (i.e. 960+ lift in units lost, which is 48 units lost in 15 raids, or 3 units per raid), demote to T17. Seems to me a difficult condition to fulfill, so if you manage to get to T18, you probably won't drop below as long as you continue playing daily.
Players who are extremely solid on offense will likely promote. Players who have some skill in defense and only lose 4-5 units in 15 attacks will probably promote also.

T19: 8200 - 9399 lift
Minimum aether per raid: 91 - 18 = 73
Maximum aether per raid: 102 (-20 = 82)
500 aether automatically regenerated + (7 free battles x 18 aether from breaking opponent structures) = 626 aether
This one, again, is marginal. You will get 7 raids in, maybe 8.
7 raids + 7 free = maximum 1400 gained on offense.
If defense loses more than 200 points (i.e. fails to achieve 18 kills in 7 defenses, which is about 2.5 per defense) after perfect offense, stay in T19.
If you manage to get the 8th raid in, defense can afford to lose 300 points, but must still claim 13 opponents in 7 defenses, which is about 2 per defense.
If defense loses completely, offense must score lower than 560 to get demoted, which would require losing 860 lift worth of units, 43 units in 14 raids which is still 3 units a raid. So if you manage to get into T19 you probably won't drop out.
It will take a player with skill well above average to promote beyond T19 unless fountains become upgradable.

Hypothetically, let's say upgrade aether amphorae to hold 250 aether and fountain to regenerate 75 per day. 6 x 75 = 450, and +250 at the start + 126 from breaking structures during free raids = 826 aether in a week
10 raids + 7 free = 1700 maximum gain on offense
If defense loses more than 500 points (i.e. in 7 defenses, fails to achieve 3 kills), stay in T19. So if fountains become upgradable, players strong on offense who have any skill at all on defense will probably promote.

T20: 9400 - 11000 lift
Minimum aether per raid: 103 - 20 = 83
Maximum aether per raid: 118 (-24 = 96)
500 aether automatically regenerated + (7 free battles x 21 aether from breaking structures) = 647 aether
At 90 aether a raid that's 7 raids
7 raids + 7 free = 1400 maximum gain on offense. Insufficient for promotion even with perfect defense results. A higher level fountain or much higher level amphorae would be necessary.
If defense fails completely and offense fails to gain 560 lift (i.e. misses at least 860 lift by losing 43 units in 14 raids, which is more than 3 per raid), demote to T19.

Using the hypothetical 250-aether amphorae and 75-regenerating fountain from T19 analyses, 847 aether available in the week.
9 raids + 7 free = 1600 maximum gain on offense. Defense would have to be 100%.
Supposing the fountain can regenerate 100 aether a day and the amphorae starts the week with 300, 600 + 300 = 900 aether, plus the 7 x 21 from free battles = 1047, which can support 11 raids. Then defense would need to limit losses to 200 lift in a whole week (in 7 defenses that would demand 18 kills, which is 2.5 per defense) to promote.
Nobody's getting into T21 until fountains can regenerate at least 100 aether a day, probably more.

Summary
T16: possibly to jump straight to T18 if you're strong
T17: a few players start to get stuck here, but most probably promote. If fountains become upgradable, definitely most will promote.
T18: probably where players sit if they're in the weaker half of average players. Unless fountains become upgradable.
T19: probably where most players sit who aren't among the strongest
T20: requires a very strong player until fountains become upgradable.
T21: unattainable in the current state of the game.
Once you gain a tier, it is not difficult to maintain unless you start losing 3 units per raid on offense.
Defense does matter, and T19 is where it seems to matter most in the current version of the game. One kill per defense will not cut it, so all these all-out hurl-at-the-enemy strategies that fall apart completely after the first kill won't be enough, especially against players that make it this far. At >5800 lift myself I can already say that players at these levels are very unlikely to fall for that.

Footnotes:
- I used the formula on Gamepedia for amount of aether regained from a battle. I'm not sure if I used it exactly correctly.
- If you don't consistently break both the enemy's aether structures, you won't be able to progress as easily as my calculations suggest.