In DNN 7.1, the ability for users to create vanity URLs was added. This feature cannot apparently be turned off (which should probably also be fixed).
But, more importantly here, there is no ability to create a prefix for vanity URLs - they are all at the site root. Nothing then prevents users from "reserving" page names that might later be useful. Also, users can make it look like their vanity URL is a real page on the site.
Imagine if google added this and I went and made my vanity URL "store" or "products" or "****" and thus you found my site if you went to google.com/store. That's obviously a terrible idea and no normal company would allow it...
The easiest fix for this would be to make it so administrators can set a prefix for vanity urls. So, instead of example.com/mike, it would be something like example.com/users/mike. This way, it's more obvious that this is a user's address on the site.
Without this, it seems dangerous to upgrade to 7.1, which is why I classified it a show-stopper.