And the Dalish are not the first elven culture. We don't even know if the Arlathan culture is the oldest for the elves.
Because the City Elves may actually feel differently on things. Like for example they believe in the Maker over the Elven Pantheon. Their choices shouldn't be limited to "believe what you want but rot in the slums" or "sacrifice your beliefs and live in the forests". The City Elves deserve more than that after all they've been through.
It's the only elven culture we know. That's not all, but it's something. You may headcanon whether you want but the canon won't change
Then they're more human than elven. Might as well cut off pointy ears. Somebody has to remember. If the city elves don't want to, the Dalish will.
No one forced them to live there. They are not technically slaves. They chose themselves to live alongside people who despise them and do not see as equal because of race. The Dalish have their own problems to deal with so helping those who do not respect what they stand for is out of this question. This is not charity
And city elves have no one to blame but themselves... And humans, of course