If they FBI did actually crack a private key, it would almost certainly have to be with a top secret, insanely powerful quantum computer that's decades ahead of what is publicly known to exist. The existence of such a computer that could crack bitcoin private keys would also be a powerful tool against every organization on the planet and their ability to maintain secrets.
I'm referring for post quantum cryptography, https://en.wikipedia.org/wiki/Post-quantum_cryptography, which would negate the usefulness of such a quantum computer at cracking competitor secrets.
Bottom line, there are much more useful things you could use this computer for, like cracking all encrypted communications of a foreign power or hacking into their military or financial systems. Using it to crack a single bitcoin key to recover a few million dollars only serves to alert all your adversaries that it's time to upgrade their cryptography.
