There are lots of examples of governments maintaining the integrity of keys over the long term. The e-Passport system is an example of that. Another would be that the NSA had the ultimate insider hack - literally a sysadmin who dumped their entire internal wikis and document stores - and yet it appears that no key material was compromised.

You can repeat "a backdoored system is no system" mantra to yourself if you like, but there are a LOT of threats that aren't governments and those are the ones people tend to care about the most.

Threats posed by government are much more profound than those posed by any other type of organisation, because governments have much less effective checks on their power.

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.

Because it involves trusting that no one made copies of the key, turning a locked door into a backdoor? Your mitigation ideas are sound though, and could also be used as a kind of rolling canary trap.

While I'm not a fan of crypto backdoors - thinking of it, aren't all public/private key pair systems a form of backdoor? If someone leaks the private key then everybody with the public key are affected? Think root CAs or app store signatures as example. They manage to keep their private keys secret quite well.

A backdoored crypto system is a no crypto system.