I do not think you can per se say, that NiKo did not improve the last 3 years. It is no always about individual skill. CSGO is a team game. And FaZe had struggles with Niko trying to IGL, roster moves, uncertainty and maybe inconfidence in played position. Do not make it too easy.
NaVi as a team is highly build around s1mple, to set him up for plays etc. FaZe doesnt play all down to NiKo.
Regarding the international team, the Astralis players also played with people from their country and did not join an international roster. So I do not see a reason why that would matter at all.
Sure, you did not say s1mple only has mechanical skill, but that is where he stands out. And his gamesense of course. Overall his capability as an individual player.
S1mple definitely has a more flashy playstyle and likes to go for more risky and hyped plays.
Yes, s1mple has a great work ethic. I do not know too much about ZywOo in that regard so I wont comment on his work ethic. But the fact he is still so young and yet so insanely good on big stages makes me doubt he will struggle a lot.
Regarding the GOAT status, I meant to say, that GOAT of the game takes more than just individual capability.