Player Drop Off and Accessibility/Diffculty

[Shistar]

I don't want to play any game where I have to guess how combat is going to work fight to fight.

Understandable, that's a fair opinion. However, the game has always been like this, and it used to be much worse, so I'm wondering why it you didn't feel it was a problem before?
Bosses often had obscure mechanics unique to them that you may not see again for the rest of the expansion, take Diabolos's portal doors from Amdapor, or the boss before having the tank kill an add to be able to draw aggro in the first place. These mechanics could get quite punishing, too.

I think the added variety of having to figure out what the boss's choice of communication will be is fun, personally. I also enjoyed ARR+ mech design best (or what was left of it when I began playing in 2020, many old dungeons would consistently wipe careless groups when this doesn't happen anymore sadly) where things are overall slower, but there's more moving pieces.

I think placing the bigger part of the game's complexity on the jobs rather than on the fight design would allow both of us to have fun: You can choose to perform a simpler version of your rotation so you can deal with mechanics more effectively as needed and fall back into the normal rotation as you get more comfortable, and players who want to push the limits can try to reach for the skill ceiling of their preferred job.

This 100%. The rewards are not worth the tedious process imo; specially when they become obsolete by the next patch or two. If they were necessary for relic weapon progression, I'd be more inclined to do savage content.

I agree, but honestly I'm quite glad savage is not necessary unless you wanna see your ilvl rise. It's a time investment I wouldn't want to do without a motivating reward, and nowadays that's the glam/mount attached to the fight for me.
Relics having multiple path options (let's say in the case of DT it'd be OC, savage or fates/roulettes as a lazy example) would allow us to work towards a goal but keep things fresh. Being able to mix and match would be fun.

It's harder to do ARR dungeons as a support than in Dawntrail.

This is so true, and I find myself enjoying older content more because the combination of the lobotomized kit (not so many emergency buttons), leftover obscure mechanics from that era and potentially a sprout tank that has no idea how to split up the pulls makes for the most engaging roulettes you can get your hands on as a healer. It's that, or get thrown into a synced Shiva EX with a bunch of people who don't speak your language + don't know the first thing about the fight and figure it out from there lol mentor roulette memes are always something.

I tried using gear from one of the recent dungeons lately in a glam set but gave up because I couldn't get colors to match.

In case you or others reading this later were not aware, you can use a search function in Eorzea Collection to filter for any gear pieces you're interested in using, to easily see how others put outfits together with it. It's not a solution to the root problem, but if it at least helps you wear the piece you wanted...

[Absurdity]

Understandable, that's a fair opinion. However, the game has always been like this, and it used to be much worse, so I'm wondering why it you didn't feel it was a problem before?
Bosses often had obscure mechanics unique to them that you may not see again for the rest of the expansion, take Diabolos's portal doors from Amdapor, or the boss before having the tank kill an add to be able to draw aggro in the first place. These mechanics could get quite punishing, too.

I think the added variety of having to figure out what the boss's choice of communication will be is fun, personally. I also enjoyed ARR+ mech design best (or what was left of it when I began playing in 2020, many old dungeons would consistently wipe careless groups when this doesn't happen anymore sadly) where things are overall slower, but there's more moving pieces.

I can't tell you definitively, but I can give you my guess.

It's a problem now because people have gotten too comfortable with bosses just doing "dodge the same 5 orange AoE patterns while it casts for 10 seconds" for 3 expansions straight.
They even went back and reworked the mandatory ARR dungeons to do exactly the same thing, or they are so powercrept that you can simply ignore the mechanic.

So most players now have never even experienced having to figure out a boss' unique mechanics from whatever obscure tells it had.
Best example is Sunken Temple of Qarn. People regularly die to doom despite there being an obvious glowing platform and nothing else in the arena, because the game no longer bothers to make them develop the skill of "look around at what is happening".