I don't really understand why the OP is taking so much flak for this thread, especially as so much of it seems to be built on a kind of childish histrionics based on the mistaken assumption that a) anyone's actually comparing Martial to Messi on a pragmatic level, or b) that anyone is especially suggesting that it makes footballing or business sense to keep a relatively unproven Martial over signing the GOAT.
But I think it's a good thread, because it brings into focus what it is that individual fans want, as fans. Not as fans trying to project themselves into somehow being Ed Woodward and therefore caring intensely about the size of our brand in S. America, or having to suppress some kind of terrible paranoia about Martial one day leaving us, as if this has any bearing on the completely hypothetical scenario that
@Pexbo brought up.
Surely there is a place of absolute, unfettered sentimentality in football, otherwise it's just another aspect of the big-business of entertainment, with all the amorality, corruption, and narcissistic money-generating pointlessness that big-business engenders.
So I get the arguments on both sides that either take sentimental joy in imagining Martial's development and flourishing at our club, or equally those that find more joy in the idea of seeing the best player of all time walk out in our colours, no matter whether it's for 1 season or 10. Why shouldn't we talk about this; surely the idea of what could or might happen is what keeps us all being fans, and this lets us look at what colour our imaginations as fans have. Nothing wrong with that.
Personally, I have no emotional attachment to Messi, so while swapping Martial for him would almost certainly be the right footballing and business decision, I don't need to care about that, being a random nobody, and can indulge my sentiment in this fantasy discussion, so I'd keep Martial.