I recently got a clear in a9s when I, as pld, in shield oath 100% of the time, did more dps than one of the new bards. (the other, also new, bard, did double that while also learning the alarum mechanic)
Did we clear it? Absolutely. Did they learn? Probably. Was it worth it? Christ, no.
Back when I was a tiny baby scrub, I hit lv50 and unlocked ex primals. Hell yeah, I thought, I'm gonna clear this new thing. And I queued for it. In df. Somewhere a mentor is crying even though this was before mentor roulette.
Ruru ex I read the wiki because I knew these were challenging encounters when synched. We wiped three times, my co-healer was amazingly patient, and we cleared. and on Titan ex, partly because of the cast problem and partly because my ping is a piece of shit and I lag like a motherf, I couldn't last more than three or four landslides in. So after like, twenty minutes of wiping or somesuch, during which I was trying desperately to put what I'd read into practice, I got kicked.
Honestly? I deserved it. I was a terrible healer at the time, and that, plus my lack of knowledge, plus my lag, meant I almost certainly wasn't worth the effort of teaching. And that will always be there for pretty much everything for some people, I think. In order to know you need to improve, you first need to be told that your rotation is shite, as they probably say somewhere.
If the community won't speak up, then it's the community. But they do, almost always, at least eventually. And if a player still won't improve, then it's on them. Pretty much the only exception I will make is a new non-pld tank in their first intstance that needs a tankswap not having 'voke, cause the game never ever tells you that you 100% require this one very specific skill. Hell I've seen healers that outright refuse to get swiftcast (!) and that won't break a run like not having voke will.