I think that's the biggest difference between the 80's/90's and the last 20 years - in those days people put out the game they wanted to make even if it differed radically from D&D. Since the OGL a lot of people have had the idea they can make money from their hobby if they "attach" to D&D and plug into the network effect etc. - some did, and some did not. However, I do think it pulled a lot of effort towards D&D instead of some other system and setting.
This is exactly correct.
I honestly have trouble following the argument in the OP for a simple reason- we saw an absolute explosion of RPGs in the late 70s and 80s, most of them having completely different systems than D&D did. Even the other games that TSR made used wildly different systems than D&D did.
I'd argue that the default assumption during that time was that if you wanted a new RPG, you had to create brand new rules for that RPG as a differentiating factor. So you not only had all of these different RPGs, you also had all of these different rules.
As opposed to what you have today, where almost every game is:
A. OGL (3e derived).
B. OGL (5e derived).
C. PbTA derived games.
D. FiTD derived games (which is PbTA derived).
Once you get away from that, there just isn't a lot of actual innovation.
As for the OP, we'll see. People have bet against D&D for almost five decades now. I honestly don't know what the secret sauce, but at this point, I'll put D&D along with Death and Taxes- best not to bet against it.