Or a contractor can take it home, against policy of course, and then it gets submitted to an online anti-virus scan.

Or it gets left on a USB key in a taxi

Or it gets left on an un-scrubbed laptop that's put up for government auction.

Or any of the other which-ways that humans get around policy limitations that don't have technical barriers, or socially-engineered to get around the technical barriers.

Or the government can leave it behind their remote servers along with their cyber weapons for anyone to wander by.

Or anyone who is willing to spend enough computational power now only needs to crack one key instead of millions of different ones.

If there's anyone who can have enough computation power to break key, then that key is weak and its size should be increased.

One can only make keys so large to be useful—my point is the same, still, that one would only have to break/find/buy/steal a single key instead of millions.

Everyone can leak the private key. That's not a concern of secure protocol. Of course there should be means to migrate to new key if that happens. Important keys are unlikely to be leaked.

>Important keys are unlikely to be leaked.

The more important the key, the more sought after it will be by hackers, especially one which could theoretically decrypt an entire countries communications. It's only a matter of time.