How likely is that this will happen, unchallenged?

Seems very likely.

Is the problem then that developers have to register?

Yes, the problem is that the developer has to register with Google, and Google has to approve them. It only "blocks" sideloading if the developer has not registered with Google, which all of them I have seen, have already stated they won't be doing this. It also gives Google the authority to block any apps they don't like, such as the ones seen recently being taken down from Apple/Google stores. As well as any ad-blocking apps or syncing apps like they've done in the past. In short, it gives them complete control of the entire platform, and the associate revenue from it.