The goal is not to prevent a Google monopoly. The goal is to have competition to prevent Google abusing its monopoly to lock down users.

There is Samsung Galaxy store.There is Xiaomi store.There was Amazon app store which failed to gain traction as it was not pre-installed but had decent number of apps. This is even before we get to open stores like F-droid.

Now their viability is a question in the long term, if Google has complete discretion over what is allowed.

Google doesn't have complete discretion. On Samsung phones, Samsung's discretion overrides Google's. On Xiaomi phones, Xiaomi's discretion overrides Google's.