Definitely agree that RDM as a support job doesn't fit. All I was trying to say is that, given two options that both completely don't fit (XIV's RDM being a nuker vs. a hypothetical XIV RDM support job), I'd prefer the doesn't-fit Support to the doesn't-fit Nuker, just for gameplay reasons. Of course the best solution by a mile is to have neither, but sadly SE seem to be intent on ignoring that.
I know, it's horrible isn't it.
I think part of the problem here is that AST is primarily a healer with added skills on top. You can choose the healer that boosts damage output, or the one that doesn't - there's a clear optimal choice. Replacing a damage dealing job with a support job is a different matter - you can choose the job that deals damage, or the job that boosts it. The end result is still just damage.
I know that's oversimplifying to an extent, but I think the principle is sound.
I agree that there's a big question regarding whether SE are capable of balancing a support job so that it "counts" as one DD job regardless of the context it's in. There are certainly ways of limiting the amount of "buffs per second" a job is capable of throwing out, but it might take some creativity, and I'm not sure SE are willing to be creative at this point. If they can balance support jobs to be roughly equivalent to another DD job, though, then there won't be any demand for support - it'll be another playstyle, not another role.