However, getting rid of the /photos prefix would be a terrible improvement.

Having the /{username} at the root of the routing logic means that every URL should either query the user database for a match or use /{username} as a catch-all fallback if no other patterns match. But this makes resolving real 404 pages much more expensive.

Concise URLs deserve more praise.

Also, when you look at a site and see URLs like /wiki/index.php/MyPage it tells you about the skill level and care of the site administrators.