GitHub manage to do it. Most URLs you'd think of are either redirects to other bits of the site, or accounts owned by GitHub themselves. It just takes a bit of planning.
You can plan every "top level" path you'll ever want on the site from now until forever? Or do you mean planning as in plan to force account name changes on users when someone's username conflicts with a new feature name?
In that case it's not really different if the usernames are prefixed to be clearly identifiable as usernames - most companies still wouldn't want any user accounts that look like official company accounts.
Haha, no it just takes forcing user account name changes.
github.com/copilot, github.com/claude, github.com/models, basically everything you can think of for the last few years has been through this approach.