I agree with the OP on few points. What I feel lacking in FF14 is the lack of freedom. Mainly, it is the way bosses are "solved".
FF14 requires ONE method of killing a boss - thus, you need to memorise the solution. This is challenging in the beginning, but once you've learnt it, it becomes boring. It also creates frustration between players at different levels of memorisation. This lack of randomness makes it initially-hard, ultimately-easy instances, which reduces its longevity.
In comparison, in FF11, perhaps because most of the bosses were "open world" (sky, dynamis, world bosses), there was no real limit to how many members can participate and it was possible to change a standard strategy to cover an individual's deficiency.
Although, were actual FF11 fights "hard"? Not really... it was just more welcoming and forgiving to individual mistakes. But I do miss the range of things one could do in FF11 as small team... like soloing sky-keys as RDM/NIN... In FF11, you set your own difficulty bar. In FF14, I think it'd be impossible purely due to the lack of abilities a class has and the world-CD, and plus the rigid instance party setting (must be 4 or 8 men for instance).
One easy solution to this problem, is to let the players adjust the difficulty of the current FF14 by free'ing the instance entry restriction. E.g. Even if it's 8 men Titan-Ex, let 6 men try it if they want to. Even if it's 4 men Brayflox, let 2 people duo it if they wish.
Also, as some other ppl mentioned, introduce a certain degree of randomness in a fight. I'm sure it's possible to "control" the randomness so that for example, 4 plumes wouldn't go off in a row (although it'd be fun if it did tbh). Part of "fun" comes from being surprised, which FF14 lacks direly.
For a long time solution, perhaps FF14 needs to release a few open world bosses so that players can choose to participate killing bosses in less-restricted style, both in terms of being free to make their own parties and creating a "solution" for the boss.
The problem with this, is that then we come back to a LFG waiting game (but already, DPS has to wait for up to an hour for instances so, perhaps that's not a big deal...).