There's another thing. As someone once put it to me, there's two kinds of immigrants: those who want to merge with the culture they're coming to, and those who simply want to relocate. The way those two groups behave is very different.
Superman can't BE a Human, but he can at least play along with Human culture and stuff. This is why he doesn't build the Fortress of Solitude in the middle of Metropolis. He doesn't want to have this air of "I'm better than you". The Fortress is very much a Kryptonian thing, but he puts it where it's not where it gets in the way of other people living their lives.
In some continuities Krypton actually had a space navy. But... these weren't Star Wars or Star Trek style ships where the basic crew might be over 1,000 people, nope... a few dozen per ship at most.
Also, in the continuity
Karsta Wor-Ul came from the fleet got recalled to Krypton for some political reason just in time to get blown up because they were in orbit of Krypton. Karsta refused to return with the rest and survived the destruction of Krypton simply because she was acting as a merc in deep space.
Oh wait, in the Lois and Clark series back in the 90s the Kryptonian space navy was the reason why they had a bunch of Kryptonians show up on Earth later. Not MANY Kryptonians were able to escape that way, but enough(hundreds?) that the race wasn't functionally extinct. But this was only a tiny fraction of the population, not even all of one city.... mostly just social elites and military members.