Mbappe is a young player, how it's make sense PSG sign Martinez as a potential replacement for Mbappe? Mbappe also doesn't care with trophy and only look for a club that can pay him as the highest salary player, so Mbappe wouldn't leave PSG.
While United they have sign Antony, moreover they have many strikers e.g. Sancho, Rashford, Ronaldo, Martial and Antony. Ten Hag is really stupid if he want to sign a new strikers.
Since Inter Milan isn't in good phase, I think Martinez should stay on there and keep carrying Inter Milan until become a champion at Serie A (at least).
I think it would still be good, Antony is at the right wing and not a striker, Ronaldo is 37, and neither Rashford nor Martial could even come close to how Mbappe is right now. I think with Mbappe's love for Ronaldo, it would be a simple sell, they would say Ronaldo is your childhood hero and he is here, and he played here before as well, and can convince him.
It's all about the finances, if they can afford him, then I believe they can convince him, but being able to afford him is not that easy, it would take nearly 200 million pounds (both player and team) at least, and that is not something the owners would like to do, it would be way too much, sure some of it will come back as jersey sales, but that's about it.