Sure it is a good feature to have, but this sort of attack is going to be targeted. You don't do it to everyone. In the unlikely event you do get caught then you can act dumb in almost all cases. Blame Pegasus or whatever...
I have no illusions about being able to keep a nation state attacker off my devices, but not having all of my correspondence shared with all kinds of third parties by default sure feels nice.
Don't forget anonymity with temporary/throwaway devices and handles combined with E2EE PFS protocols can be very powerful to thwart such surveillance.
Except that if your target finds out, it would undoubtedly increase their paranoia level, making future attacks against him harder and compromising future operations. There's a reason why law enforcement keeps wiretap warrants sealed.
The fact that anybody can verify keys raises the stakes of attempting a MITM attack significantly and thus protects the majority that doesn't verify keys themselves.