The number is the user_id and it is the key piece of information required to keep the URL.
Technically you could achieve something with Route Filters but it would be very much a manual process, e.g. if it was just one account you could do this:
But, I'm sure you wouldn't want to do that manually for all your users
Dang it. I would really love if XenForo didn't use IDs.
Would this ever be a feature?
If there would be two threads with the same name, this could be how XenForo would handle it:
First thread name: My Awesome Thread
First thread url: /threads/my-awesome-thread/
Second thread name: My Awesome Thread
Second thread url: /threads/my-awesome-thread.2/ - /threads/my-awesome-thread2/ or anything like that.
I should also note that just because they don't have the same user name doesn't mean that they won't have the same URL. "Mike" and "Mike!!!!" would both get URL "slugs" of "mike".
Plus it totally falls over with user renames, so it's unlikely to happen.