Fans on here love Rashford and Martial, and some wouldn't want to swap one of them for Mbappe. But if PSG were stupid enough to offer that trade our board wouldn't even hesitate to take it.
Just accept that you've had a horror show in this thread mate. £50m for Pogba ffs. As others have said, the only one of those players that you mentioned who wouldn't be instantly traded without a moments of hesitation by their clubs would be de Bruyne. Form is temporary, class is permanent. A bit of bad form (which was being significantly exaggerated by our manager and the media) doesn't wipe out everything he has achieved already and how ridiculously talented he is.
I accept that valuation was way too low, albeit with the benefit of hindsight. At the time, I expected Mourinho to linger on for several months, and Pogba to be disposed of in some sort of fire sale in January. Had that happened, he would have been damaged goods - not in the 1st XI, attitude questioned - and that would have impacted on his selling price. Looking back, maybe the board felt their hand was forced as they didn't want to offload their marquee player to placate a manager who would have been gone at the end of the season anyway.
On Mbappe (a bit off topic, I know), I probably would accept him in place of Martial, but not Rashford. As you say though, it's not something PSG are likely to offer anyway.