Making heals more required in general casual content would fix that, even if you had "healer specs" you can balance it in a way that would hurt the general DPS of a role speccing into healer, I wouldn't expect a massive "skill tree" Potions from bozja like content are a decent example, you who sacrifice damage or tanking ability for healing output (you could even make heals tied to a certain hardline healer spec that would mean you'd have to have decreased damage no matter what, if you wanting healing ability, Obviously this would need a lot more work and would likely just be a pain to balance, but I personally enjoy a job being able to fulfil more then one role and only having one set way of playing.
Right now Healers contribute around 50% of a DPS damage, which is massive compared to something like Wow or other games for a Job "Healer" Should healers, generally be more heal focused yes, that's their purpose in a game generally, that's a problem with fight design, Healers aren't in a good state, they need to either up the healing requirements or rework healers in general, the current design makes healers pretty useless in content that 99% of the playerbase is playing, The problem is when you make content where a certain role isn't required or makes it significantly harder to do without.
I think you're blaming poor game design choices that ruin the healer on any actual Idea or innovation on how healers could work (would be pretty hard to balance everything with the current direction of the game so it's only a dream for me), My idea to let more jobs have flexibility isn't really fleshed out because I know they will carry on the current design on healers, I don't think they would go into any actual options in the game because it would be harder to balance, I don't think it would ever be implemented and like I've said just because I enjoy diversity and more different ways to play a different job, doesn't mean you have to agree.
EDIT: also yes sometimes some specs are more "meta" then others but if you compare different jobs to others theirs also better meta picks, theirs always going to be a "optimal" and "suboptimal" way of playing any game, making everyjob the same isn't a good fix (every change right now has been fit jobs into burst jobs) I rather a clunky unbalanced game with options then a boring game where every job type plays the same.