I think they're both right and wrong.
If D&D:HAT is a good movie, and well-regarded, it will help cement D&D as a "solid IP", not of the Star Wars/MCU or even DCU level, but perhaps approaching the level of more mid-grade IPs, and would certainly help prevent any stigma returning to RPGs.
But honestly I think we're past the point where stigma of any kind can really be an issue for D&D, except possibly the dread "That's fun but it's for old people" or "That was fun when I was a kid". D&D is currently pretty well-positioned to avoid that, given the core audience seems to be about 15-29 buuuuuuut all it would take would be an '80s-like situation where a lot of people just drift away from the game for it to perhaps acquire that stigma. I know loads of 30-40-somethings currently teaching their kids D&D, but will those kids go on to play it as adults, or will they leave it behind as a game for kids? I think that's in the balance, and I don't think D&D:HAT or sequels will actually make much odds to that.
If it's a failure, things do become more uncertain, though because WotC have limited financial exposure there, I don't think that would cause big decisions to be made. I think it would take both a failure of the HAT and of the 3D VTT for big decisions to start getting made. I'd say the 3D VTT is far more likely to fail, too.
What would those big decisions be? Honestly they could go in a lot of different directions. The only commonality is Hasbro/WotC are likely to reduce investment in D&D and D&D-related products. History shows that can be a positive and a negative. My personal suspicion is that both fail, Hasbro will essentially want WotC to basically put D&D into "maintenance mode", which will be kind of good for anyone who likes D&D as it is at that time, but may set in stone issues people aren't fond of, and with the new OGL, may have damaged the ability of 3PPs to help out. It's also quite possible Hasbro would sell off either WotC or just the D&D IP, which could very well or incredibly badly depending on who bought it. Almost certainly it would be a videogame publisher which buy it. History tells us that doesn't go well - c.f. the fate of White Wolf (CCP really wrecked it and Paradox have been more gentle and positive but haven't achieved much).
There's another angle too - if HAT succeeds well (and the TV show does), but the 3D VTT Is a miserable failure relative to investment, it's actually possible Disney might buy D&D as a movie/TV IP. They'd probably keep the TTRPG going, but might make some weird changes.
Personally I'm inclined to hope it does well, because whilst I'm not hugely fond of the IP of D&D as lifestyle brand, I do think it would be better for the hobby, including non-D&D RPGs, if that happens.
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For some, their life takes a different course that is more meaningful and fulfilling. Others indulge in superficial displays of youth, denying their age. Which path will D&D choose?
