I think the second point is the biggest for me: it’s almost like Canonical wanted to have a single dominant store for apps, as the ecosystem they are building supports only one. And, apparently, that one server is also closed?
So if you try to make an alternative source and give instructions to people how to configure their snap installation to use it (I found this information very hard to find for some reason…), your “store” probably won’t have the same packages Canonical’s has, so users won’t be able to find the packages and I imagine updates are also now broken?
Contrasting this with flatpak: you just install apps from wherever. Or from flathub. Or your own site. Doesn’t matter. No business incentive behind—built into the tools—to make everyone use flathub.org.