I have news for his opponents: if they think he’s wrong, they can declare war on Turkey.
They aren’t offering an alternative because they’re opposed to anything the President does.
The error Americans make is to think the Kurds are monolithic. They are divided by tribe, by politics and by nation. Historically, they have never been able to unite, and they have not done so recently. We have supported them both politically and with arms. If some thought America was going to be in Syria and Iraq forever (and some factions have not been so foolish) they have misread President Donald Trump.