Sony could have imposed a fee to unlock Other OS

No, that's extortion, which is also a crime.

First of all, I think every "pay to unlock the capabilities of the hardware you already bought" is literally criminal (and by "literally" I mean "actually literally, not figuratively, and the FTC is failing to do its job by not prosecuting it") no matter how much it gets normalized. Second, in this particular case it was unlocked from the get-go by default, so Sony would've had to forcibly lock it first and then try to charge for it, which would've been somehow even more blatantly criminal!

The real alternative here would've been for Sony to take the fucking L and comply with the law. They could've revised future units to not support OtherOS (as they, in fact, did), but forcibly removing it from units that had already been sold was absolutely beyond the pale and should've been stopped.