Yes and No.
I think there's definitely an argument to be made that D&D being the gateway drug for the vast majority of people getting into RPGs results in it's 'runoff' finding other games. …
There's probably an argument to be made that because 5e is such a roaring success there's more people/groups who won't be able to comprehend playing anything else, especially with things like D&DBeyond becoming a thing. … If WotC start really doubling down on that kind 'contained eco-system' somehow, at least in the longterm, we might see Games Workshop-esq issues arise but I doubt it'll come to that for a while, if at all.
I Believe that with 5e the runoff is proportionally less than it probably has been in the past.
The top 2 selling rpgs are D&D and D&D’s clone.
IMHO we are starting to see the long-term effects of Dancy’s Brilliant OGL policy.
D20 uber alles… Which has benefited WOTC enormously!
There is D&D, and then the rest of the hobby. D&D is doing Great, (And those who make 5e compatible product) but I don’t think the trickle down to other systems is in the proportions that it was in the past.
In the mid 90’s, when you went to a gaming store there was more variety on the shelves system wise than there is now. So a Casual fan walking into a gaming store got exposed to different RPG’s just by showing up..
Now it’s all online, and there’s tons of RPG’s out there for sure. But you have to know where to look.
And the current 5e ecosystem is not big on DIY rules mods for 5e, let alone different RPG’s altogether.
As for GW type issues... Don't give them any ideas!
WOTC could throw their weight around in the RPG market far more than they have been...
It's the same issue though. VTTs greatly constrict the range of games that can make an impact, due to their level of support.
And it's a vicious circle because only games that have a large enough group of players will get significant support.
I’d be very surprised if for 6e WOTC doesn’t pick one of the VTT and do a 'D&D Beyond' type deal with them making them the Exclusive VTT for 6e D&D.
Every new book/adventure Drops only on their WOTC approved VTT. Case and desist letters for the rest.
I'd also do a deal with one of the 3d print figure companies - make them exclusive. Then have a way when you order your custom figure model you also get a token file to put the 3d version on the VTT as well.
Exclusive VTT License + Subscription model = Rake cash...