What did anyone think the end state was going to be for the Kurds? A new state would have been highly unlikely to have won UN recognition as its leaders would have been portrayed as US-backed opportunists. Even if by some miracle it had emerged into statehood it would have been in a weak strategic position and lucky to last a generation before getting carved up between its neighbours. It would have needed to be propped up indefinitely, to what benefit for any external power?
Perhaps their best chance was to pursue a settlement with Assad for an autonomous region within Syria, affording the protection against Turkey that was so obviously going to be needed. That would have meant letting the Syrian Government reassert its territorial rights east of the Euphrates straight away after the defeat of IS, which regrettably they didn’t do.
I can’t really fault Trump’s determination to reduce the US’s exposure in the Middle East. If the US is to be criticised, it would have to be for giving the Kurds reason to think that they might be able to carve out a state. They have now overreached, just as they did in Iraq.