yes, it can have these network effects, but as long as it opens up access and benefits customers, you will have a hard time making a case that it is anti-competitive rather than just offering a superior solution for buyers.
It is not WotC’s job to hobble themselves so Roll20 can stay in business. Nor is this the only way in which WotC can make competition harder for Roll20, but at least this way it is beneficial to the users rather than detrimental. As long as it benefits the users, I am mostly for it, even if I can see the potential issues lurking in the back.
yes, they probably would not want local storage, even though there could be security measures in the file (ie bound to a DDB account, with ownership being verified if you want to import it in another VTT), so it cannot be freely shared. I said this one is the harder sell