Warden is still a corner stone of pvp BRD for the reasons you mentioned. And it's still a niche skill that's been used in anabaseois for 2 fights, which was ultimately fun for the BRD, but in result extremely gimmicky as in, it was kinda useless anyway since the fights were designed to play around it in case of no BRD in party. Ideally you'd have this matter more in a fight where BRD's unique identity would help and be appreciated, but there it was just not the case. Healers could do it too. In P12 p2 it was kinda handy to allow your glare mages to cast more glares I guess.
Curing Waltz is incredibly powerful and so is Improv to a lesser degree yeah. The latter even used as a weave actually heals quite a bit over time. It's definitely because DNC is more recent but I also suspect it is also no stranger to DNC having been expected by the playerbase before ShB as a new healer, which it ultimately wasn't, but I'm pretty sure it also made sense in the devs mind. Even in pvp its healing kit is a non negligible part of the job.
I do not know what they need to do with rphys on the support side, but as long as everyone and their moms have mitigation, and as long as resource support isnt a thing anymore, I don't think that doubling down on defensive support is suddenly gonna make the role better. You don't take it for this now and will probably not take it either unless the defensives are literally tank or healer LB levels of good. Or... if it's a raise. See DNC for example: its support is pretty good, and yet, why would you take a DNC right now beyond the 1% party role bonus? (hint: there is literally zero reason to take one)
It always boils down to all the facets of the game they axed and how barebones they have made the battle system today. It just doesn't offer any specific and important/mandatory niches to fill anymore for the role to be relevant in its current state. The only way to make it relevant in the current system is to make it like any other DPS, with average support tools and a good dps output. It sucks for identity, but that's the sad reality of the current battle system that doesn't accommodate anything else.