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.

If you delete the photos path element they do actually have a fallback resolver that redirects you to the appropriate path under photos.

E.g. flickr.com/mwichary/sets/72177720330077904 does take you to flickr.com/photos/mwichary/sets/72177720330077904