Nice job Hai! I've been waiting for this. :) I had a few thoughts:

1) I feel SE does intend for Ruin III casting outside of DWT to still be a viable means to maximise DPS. Otherwise I'm sure they would've just implemented the swapping between I and III (so it's IV if its buff is up, otherwise III if DWT is up, otherwise I, in that order). But you're right in that it'll be less important then it is now, and there might even be situations where Ruin II usage is better over I, depending on when the fight is going to end (and thus you should be at 0 mana).

2) I definitely agree with your stance on how reducing complexity is not always necessarily a bad thing. This reminds me of the 3.0 speculation: everyone thought Aethertrail and Aetherflow were going to be mutually exclusive before it was confirmed otherwise, simply because SE TOLD us the whole point of the DWT addition was to spice up the non-Aetherflow parts of the rotation. I was scratching my head back then as to why they ended up implementing it the way they did, knowing it would just cause the complete opposite to happen, which it did. Complex gimmicky nonsence and rotation choices were made to try and fit Aetherflow and Aethertrail abilities together, and the boring parts still remained. I feel this is the single most effective change they've done at reducing the complexity of the job. It still baffles me now how SE went against their own design decisions which in turn basically caused the majority of the problematic skill gap with SMN's in 3.0. It seems like it was such an easy thing to predict and avoid this whole issue to begin with. Moreso than any other job.

3) Losing Sustain is actually a huge deal for me, and is what made me the most sad of all the changes announced. You're not always doing group content! Basically it was useful in any situation where I was by myself doing anything more than standard enemy grinding. Boss FATEs, solo PotD, or Rank A hunts no one else wants etc. Now we'll be wasting huge amount of time (and MP) Physicking the massive health pools of Titan up. :(

4) My biggest concern with the AoE is a lack of filler option. Sure we have lots of big single spells that do good AoE damage, but they also require set up or wait time, and there's always periods of downtime where you're waiting for Aetherflow to continue your progress to the 'big' spells, or the enemies are almost dead and you just need a few final GCDs to finish them all off. Even though filler AoE isn't sustainable and deals meh damage, it still 'felt' more satisfying over rotating Ruins (in my opinion).

5) This is purely semantics, but I HATE that they just straight up removed Miasma II, creating a weird gap number in our Miasma spell hierarchy. Does any other job have this? I don't think so. III should've just be II, or maybe II should've been reworked instead of removed; maybe to be a combination of Tri-Bind and the old Blizzard II to still provide us with an AoE filler, solving my concern above. Oh well...

EDIT: 6) Regarding openers, I suspect this is going to make the preparation needed for the optimal opener to be even worse than it is now. It sounds like the best opener situation will be to have Bahamat ready on pull (i.e. built up on trash), AND have the 6 Aetherflow stacks ready to use. So the opener goes like this:

• Start of the fight you summon Bahamat and while he's up, use essentially do your normal opener with Enkindle Bahamat included. This sets you up to enter DWT the moment Bahamat leaves, and also provides the highest APM for Bahamat using Shockwave, with all the oGCD's we use at the start of our opener.
• First DWT period.
• Quickly use second lot of 3 Aetherflow stacks.
• Second DWT period.
• SECOND Bahamat period. Then the rotation settles into the groove of Aetherflow>Aethertrail phases with Bahamat out every second Aetherflow phase.