This is a really good question. What are the actual practical fears?
First, they could abandon it when it's no longer popular like they did with all the 4th edition material. You could spend a lot of money on it, both as purchases and subscriptions and they could take it away. This isn't hypothetical. They did so with all the 4e tools. They did so with Dragon+. They killed all the articles they had posted on dnd.wizards.com. Granted you didn't pay for that stuff but clearly WOTC doesn't care very much about preserving old material.
The whole system can degrade as more sources get added. I argue it's doing so now. You can't, for example, filter out sources in the character builder if you have access to those sources. Everything shows up. Yes, I know it has those checkboxes to allow certain content in a campaign but they don't prevent options from showing up in the builder. This got worse and worse over the years leading to the huge criticism of Silvery Barbs, a spell that otherwise should only matter if you're playing Strixhaven yet everyone seems to have it and it's the bane of many a GM just because its in Beyond and unfilterable.
The whole system can degrade in other ways too. Search becomes harder to use because you're searching everything – including things you don't have. There's no way to search on just a single source. The more you add new products, the worse it gets and yet adding products brings in revenue so its likely prioritized over fixing core features.
We have no idea how they're going to make 2024 and 2014 material work side by side in the character builder. Are they going to abandon 2014? Are all 2024 options going to be listed alongside 2014? Will there be two different character builders? Whatever they choose, you're stuck with it whether it's your preferred way or not.
They can change their business model – moving from purchases to subscriptions. If that's profitable enough, maybe they stop selling sourcebooks on DDB and only rent them to you.
They could remove ways to pay for content like they did for buying individual class features instead of a whole book.
They could downsize the staff which means the platform you've invested your time and money begins to degrade even further. Bugs keep popping up. New bugs show up. No new features get added. Old features break.
Right now we're in the honeymoon phase of D&D Beyond. Profits are up. Attention on the brand is great. Executives love to charge rent. What happens when things go down? How will they behave then? What will they do with the platform?
Whatever they decide to do, you're along for the ride.