I would go as far as to say everyone that wants to remove the trinity has never played a game that was originally designed with a trinity in mind actually remove it. And thus never saw the actual consequences of such a thing.
Some of you have likely seen me say this before, but I came from a different MMO known as Dragon Nest, which didn't have a strictly defined trinity, but all of its harder content was still designed with it in mind. After a while, as the game kept pushing out new classes at a stupid rate (along with the infamous Korean F2P MMO Power Creep syndrome), we eventually ended up with true hybrid classes that did everything better than the purer classes. It turned into an arms race of party buffs and overall utility.
("Hey, let's release a new healer which we ADMIT to releasing specifically to compete with the one unofficially designated 'pure healer' class in raids, with super mobility, double the buffing strength of said pure healer, have UNLIMITED RANGE HEALS, -triple- the healing strength of the 'pure healer', have DPS that rivals that of actual pure DPS classes, and summons designed to circumvent the bad AI that plagues a different summon-based class. And we'll justify this by saying that they have to remain still and meditate in mid-air to use their buffs, but it won't even matter because all of their damage is frontloaded into two attacks, one of which can be launched from a mile away and still hit because of absurd AoE, and the floating will actually be a huge advantage because bosses for the next year just coincidentally happen to spam ground-based attacks like no tomorrow that screws over how the 'pure healer' is designed (support through summons that die as soon as they're sneezed on). Oh, and let's make this a branch of the Assassin class."
I mained the 'pure healer' during that time period and I will never ever freaking forgive the developers for going down that route. And that wasn't even the worst of it, that class was so absurdly broken at release that for the next half year, they basically 're-balanced' all classes around its existence (and failed so badly to the point where there was a 'light elemental meta', in which deviating from it in terms of raid composition would actually lead to as much as a 10-20% DPS loss for omitting even a -single class-). Thankfully the devs have learned since then and new classes from that point on have actually been relatively balanced in comparison, though it also resulted in an across the board massive nerfing of party buffs, many being outright changed into self-buffs.)
As a result of the above absurdity (accompanied by a huge population drop worldwide as people lost faith in the developers), the developers actually redesigned the entire game into a strict trinity - and for the first time admitting they would design the game around the existence of one - and class balance in comparison since has been better (outside of a few outliers in the DPS department, which especially salty people that formerly mained the hugely broken classes from the prior time period - and thus hugely nerfed after - use to push a narrative saying that this new direction has failed).
It is an extremely bad idea to wish for such massive changes without first thinking about how it would affect the rest of the game, from encounter design to subsequent changes in community mindset towards classes.