Indeed but it comes down to a simple concept:

It's what sells in this day and age of MMORPGs.

So while they tried to switch it up, it's why I liked what they started doing with bosses in XI in it's later life despite limitations of the engine. While you tank and spank, they make it so you can't just do it straight up by changing the battle flow throughout, i.e you have some bosses that throw up a magic shield which gives it a silence/paralyze aura which causes you to shift your tactic which is something they did heavily in FFXIV with some bosses:

Garuda changes her attack patterns and spits out clones including battlefield hazards after Aerial Blast if you skip no phases (how SE intended the fight to be). Comparatively, in FFXI if you fight "Seed Crystal" in the addon you have the crystal focus on your tank and it will occasionally spit out clones of a party member that has to be dealt with or you're eventually swarmed with clones.

Chimera if you incap the tail it loses the ability to use scorpion sting for awhile much like in XI if you incap hydra's heads it loses a lot of defense and regen power.

So even if it's "stale mechanics" when you really analyze it, it's more about if the fight itself is interesting especially in regards to storyline fights. People want to fight Bahamut and come up with crazy speculation of the fight, but within a realistic expectations it's likely going to be another form of tank and spank with various ways to help/hinder the fight, i.e it's guaranteed he'll use one of his Mega/giga/tetraflare attacks but it just comes down to when/how. In Abyssea in XI, there's a monster you can't physically attack at all due to him "flying" and even monsters(in sea for example) who travel above/below the ground untargetable.

So really you could only do so much within design without turning a game into something it's not, like if a boss requires a ton of platforming to accomplish it's no longer the same game you thought you were playing lol. What it really comes down to though:

just jam on your strongest WS or spell and it's a sure win.
This is up to how SE designs the game and how players play the game because this is why people don't like seeing ARR coming to a 'spamfest' because that's the type of encounters that end up being introduced or the typical "must kill minions before main guy is killable" you see in many MMOs of today.

I guess what I'm trying to say is that they can include situations which require players to think critically within that framework if that makes any sense.
And as said, if this happens people will complain it's too hard or "not player friendly" and we're back to square one.