occam
Hero
The rest of your post raises several reasonable concerns, but I just wanted to address one point, regarding D&D Insider (for 4e). No one bought anything on DDI; you paid for access. When DDI went away, did it suck for people who might've wanted to continue playing 4e? Absolutely. But you didn't lose anything you had paid to own. (Any characters you had made could be exported to PDF, so those could be retained.)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.
D&D Beyond is a whole different story. I've actually purchased things on DDB. If WotC ever discontinues paid content on DDB without allowing a way to keep an offline digital copy (which isn't available today), there'd be hell to pay.