I tend to agree with both parts of that statement. But what is the worst case scenario for the hobby. What is the equivalent of revoking the OGL that WotC could do with DDB? Sure, WotC can enshitify DDB over time and make it worse for for users and third party publishers once they are invested, but this would just drive both of those groups to Demiplane or w/e DDB competitors exist at that time. It would inconvenience people, but don't think it would deter gamers from playing or creators from publishing.Adding other 5e publishers' material to D&D Beyond increases WOTC's dominance in the overall TTRPG hobby and they've proven they cannot be trusted to act in the overall hobby's best interests.
I think there are a couple of separate issues here:
1) is it bad for the hobby to have a single, dominant platform used for accessing content?
Yes and no. There are obvious conveniences for users to have everything in one place. Publishers with access to the platform likely sell more.
The downside is the potential for enshitification long term. Do I trust WotC to protect the integrity of the platform? Not really - but I don't trust ANY company to do the right thing for their customers if they think they can make more money hosing them. So I see this issue the same regardless of whether WotC controls the platform or not.
It would be great if you could transfer purchased content from one platform to another, but I think this is unlikely. Amazon Music is not going to let you play an album just because you bought it on iTunes, and Roll20 isn't going to give you access to a module because you paid for it on DDB. I think this is just the reality of the digital world, and while it is not ideal, I don't know that it hurts the hobby overall. I don't use DDB but have purchased hundreds of dollars worth of content on Fantasy Grounds. If FG went out of business and turned off its servers tomorrow that would suck for me, but I wouldn't stop gaming.
2) Is it bad for the hobby if the dominant platform is owned by a company that is a competitor in in its own marketplace?
While it is obviously bad for individual publishers who don't have access to the platform, I am not sure it really hurts the hobby as a whole. I guess you can see scenarios where WotC tries to block direct 5E competitors like ToV and A5E - but if you are buying those games (I am!) you are likely doing so as a hedge against WotC shenanigans anyway, not because you want to use those games on WotC's platform.