I should clarify: I have, in the past, said, "Man, why would I ever use this product?" only to eventually find that I genuinely like it and would, in fact, pay for it. A minor example: when I bought my current car, the one I wanted also happened to have a remote start feature. I thought that was a weird gimmick that I would never use, but I have found that I actually really do like it.
I am not above learning that WotC does come up with some service that I actually would pay $30/month for; they may have some genius innovate a thing I never imagined that I nevertheless agree is really worth it. Or you may have a thought as to some thing they could provide that I might, upon hearing about it, agree is something I, too, would pay that much for. But so far, I haven't heard of any such ideas. The thing I am willing to be proven wrong on is, "I don't think there's anything that relates to my playing of D&D that D&D Beyond could offer that would make it worth $30/month to me."
Perhaps. The biggest trouble I foresee is that I have zero faith in WotC's ability to make those actually functional and worth that price point. Especially when their first move is to try to eliminate competition by making theirs the only service you possibly CAN go to.
Streaming services compete, today, based on exclusive content more than on anything else, leaving Netflix and Youtube as the only two with actually reasonably pleasant streaming interfaces / software. Most of the others are adequate at best, with lots of clunky design choices that make it hard to navigate them, and some even have really, really bad programming that makes it slow to respond and/or otherwise very difficult to navigate within a given episode or movie. And they don't have any competition forcing them to do better, there, because what they feel they're competing on is content, not on quality of their actual streaming service, itself, as a streaming video tool.
WotC has never shown themselves to be a leader in digital tool design. And if they were planning to make something that was that head-and-shoulders better than everything else in terms of tool design, there'd be no need to hedge out competition based on content. They still could, of course, but they wouldn't need to take draconian steps; the 3D VTT would stand on its own, for example.
Maybe - maybe - an MMO-quality interface with the ability to customize it in turn-based pseudo-real-time would be worth that $30/month fee...for ONE player in the group. Or one "party account" they all access and pay into. But even then, I'm not sure, and certainly, I lack faith that WotC has the know-how to pull it off in a way that makes it worth the subscription fee.