@SlyFlourish, I understand your concerns and I like your list of "ought-to-do"s.
OTOH, I overall think that them offering 3PP on DDB is a Good Thing for the market.
It's not that I think you're wrong. It's that I think that it's BOTH beneficial AND has the downsides that you mention. The only difference between me and you is that I think that it's ultimately BETTER to do it than it is to not do it.
I just think that they should offer a LOT MORE publishers on DDB (and some top-rated DMsGuild stuff too.
AND, I think that DDB needs a UA overhaul. Specifically, it needs some of its incomplete areas completed, and it need the Content Toggles that you've described (among many other QoL improvements). If they can make some of your other suggestions work when they do that, that would be good too.
As far as WotC being good shepherds for the industry goes, well, that's also mixed. They could be
worse than they are, and they could be
better. I think that they're trying to do it well, but the nature of corporate BS makes it so that they sometimes fail (and fail hard) due to those sorts of pressures. That's a bigger problem in the world (IMO) than it is in our industry. WotC is relatively benevolent by world-corporation standards. Heck, they're better business partners for my Comic and Game store than Marvel is, for example!