Sorry was in hospital for a few! but to answer
I don’t think it would, to be honest. It’s hard for people to get into just normal dungeons and heroics already. Everyone is doing gear checks (from what I’ve seen on forums/what a group I was in was doing with DPS pugs) to get into H+ as is. Other option is hoping there’s a naxx group, but also from what I’ve seen is most Naxx pugs that have been running are running GDKP. Which, a new player isn’t going to have the gold to do.
SO! I think, and obviously I don’t know this for sure since H++ isn’t out just yet, people are going to treat it the same way they currently treat H+ (currently expecting some bis naxx gear at the very minimum/not accepting fresh 80s). Most people look at dungeons like “I want the most time efficient group possible”, which then leads to “Let’s get the most geared group possible so it’ll be nice and quick.”
I genuinely believe adding H++ (H+ also, but a bit too late for that haha) before even, at the very least, considering adding RDF is a mistake. 100%. It will just alienate any new players/undergeared returning players even more. Obviously one can argue forming their own group, but I’m sure we all know people are willing to leave groups if they feel like their time is being wasted/if things aren’t moving at the pace they want/if someone dies, and that people are less likely to join a group if it’s already cleared half an instance.
Don’t get me wrong, I don’t think it’d add to the population like at all. I just think it’d give people an incentive to play the game vs quit because “bored”. Moreso, I think, preserve the same numbers. Or at least slow down the decline of those who just choose to not log in. Would be something to spam when bored, something to use for leveling. Because right now the game is log in, raid log. Maybe do dailies. Run laps around dalaran. AFK in Dalaran. I think, really, only a small population actually plays the game outside of that (I have nothing to back this up, but that’s how it seems). Think of RDF as kind of like life support for someone that’s comatose, but in game form. Something to keep players involved and active.
Oh, like if they came out with RDF and H++? I don’t think they’d put H+ (or H++ idek what to call them tbh) on RDF. If they actually put RDF in game, I’d like to think they’d be smart enough (I know, I’m giving Blizz a lot of credit where it’s really not due) to only do RDF for normal and heroic difficulties, and keep the modified heroic difficulties to the LFG tool.