[wumpus]

This short essay is good philosophy, but doesn't seem very connected with reality. No, search engines and browsers don't permanently cache permanent redirects. Yes, you should probably use unique usernames instead of reusing them. The second fully mitigates the first, for this particular situation.

[mort96]

From some quick googling, everything I can find claims that browsers permanently cache permanent redirects. The MDN also says that a 301 causes search engines to update the link in their index.

Don't just make claims, support them. Link to anything which demonstrates that permanent redirects aren't cached permanently.