You can always mitigate the "failure for everyone" scenario. Plethora of government keys, for example, or rotating government keys, or rapidly expiring government keys... Anyway, they've managed to keep stacks and stacks of information secret for decades and decades. It seems like it could be managed. No one has stolen the cert for *.whitehouse.gov.

Sidenote, why do people call a second key a backdoor? To me, backdoor suggests exploit, not used-as-designed.