1) This is already the reality of any content in the game, it becomes easier over time. Mount farms? just wait till end of the expansion, or wait till next expansion for unsync. Same for raids. Same for most glamours, minions, mounts being shoved into next expansion's lockboxes. Same with moogle tome events having a ton of rare rewards in them. Relic steps become easier. Yet even knowing that stuff will become easier over time, the majority still farm stuff in the here and now. Why? because its content to do in the present, and because they can enjoy the reward far, far earlier than those who wait. That's the point of FF14's content design. Things always become easier if you're willing to wait potentially years, it's the tradeoff between how much you desire a reward and how much you're willing to wait. This Modus Operandi has been around since ARR, and it hasn't 'destroyed the game' in the slightest.
2) Whether you could do it in one afternoon or across 14 weeks, it's the exact same amount of work for the Doman Enclave, how would it make anyone feel like an idiot? The quests haven't changed, you'll still be reading/mashing through the exact same amount of message boxes as anyone else, talking to the same number of NPCs as anyone else, etc. In fact, it's arguably more work if you could do it in a single afternoon since you'd have to go through the effort of buying/making/etc all the items needed to fund the Doman Enclave's box in a single day, rather than just passively picking up items while waiting week to week. There's also just the point above that I got to enjoy the doman enclave's rewards for literally 4-5ish years now ahead of anyone starting now, even if they made it doable in a single afternoon. That's already a titanic positive and far from idiocy.
3) You honestly think that the weekly stuff that takes like 10-20mins to do and then you have to wait an entire week to continue is keeping people subbed? Most of the old time-gated content is stuff you can finish all of it under and hour or two a week. After that, what's keeping that player engaged for the remaining 166 hours of the week? I can guarantee you, that old timegated content isn't keeping people subbed if they weren't engaged. They'd just continue it when their sub is next running, not paying $15 a month to spend 2 hours a week on the game doing old time gated things.
Not to mention, half of the old time-gated content is irrelevant anyway - the mounts from the ARR beast tribes can be bought from any single moogle trove event in 1/100th of the time it takes to rank up them normally, making the time gate on ARR beast tribes even further pointless.
There's a time and place for time gating content - when its current, on things that could possibly generate infinite amounts of gil for little effort like leves, etc. Old content that's already been so extremely nerfed and/or had its rewards already chucked into other parts of the game isn't it.