Gotta love the disgruntled WoW fans having to come to a thread about a game they don't play attempting to put it down because other people like it. I can't blame them though, this must be more fun than the boring grinding of the same stupid never ending AP/corruption/mythic/pathfinder/echoes/cloak/essence/whatever garbage/etc systems they cram into WoW to make people waste time and stay subbed longer doing menial crap. I hope the game gets fun for them one day. Anyway.
I mean, every raid boss in every tab target MMO is literally a scripted encounter. I can't think of an MMO of this style that has bosses with adaptable AI that behaves differently every encounter or changes based on player behaviour. The most variation that ever happens is RNG picking a different player for a specific mechanic but that's that. I'm not sure what the issue is, maybe what you are looking for is PvP. Though FFXIV is not the place to go for PvP if you want to go hardcore at it.
And yeah, Jobs are predetermined. There's one correct way to play it and no variations. I mean that's true for every MMO as well, there's a specific build that will be the best performing one but at least in other MMOs you get to pick an under performing build if you feel like it for whatever reason. There's no options for customization of gameplay in FFXIV and I doubt there'll ever be since this isn't a raid or die game. I share in your disappointment in that regard.