The argument is that the result of that is a plane that isn't good enough for any of those roles, but is more expensive than dedicated alternatives. It will lose a fight against a true 5th generation air-superiority stealth fighter (an F-22 or equivalent). It can't support ground troops as well as an A-10. It is a more capable replacement for the Harrier (though many are sceptical that the USMC doctrine around that has ever made sense), and maybe it's good enough to replace the F-16, but it's supposed to replace all four of these (and will cost more to boot).
Well, the F-22 has been in service for over 15 years. The J-20 is probably not quite an equivalent, but I wouldn't want to count on that. Su-57 numbers might be too small to matter, but again do you want to count on that? The F-35 isn't equipped to fight against a peer adversary, so you're essentially relying on the geopolitical situation to continue to be that there isn't one.
