The real danger of a Syria withdrawal for the Bolton crowd is that it will force the US to reopen relations with the Assad government, especially on counter-terrorism & ISIS. 
Why? Because the YPG will patch up relations with Damascus as soon as US troops pull. They will do this to forestall a Turkish invasion. Syria, the YPG, Iraq & even Turkey are all interested in destroying ISIS. They will begin to cooperate on counter-terrorism to this end.
As Trump said: “You have to remember, Iran hates ISIS more than we do, if that's possible. Russia hates ISIS more than we do. Turkey hates ISIS, maybe not as much as we do. But these are countries that hate ISIS. And they can do a little of the fighting in their neighborhood."
If Assad and the YPG begin to cooperate on counter terrorism (CT), Washington will have to follow. This, I believe, is what really infuriates the Bolton crowd. US policy has been to turn out Assad, not to reopen relations with him.
It would be final blow to Washington's anti-Syria & anti-Iran policy. It will be a blow to Israel. For it would mean that the world will slowly accept Iran's new security architecture for the northern Middle East as the best form of counter-terrorism and security.
The only long-term C-T policy for Syria and Iraq is strong central governments in both that have capable police forces. The US is trying to substitute a Kurdish run state in north Syria for the Syrian state. This is opposed by all the neighboring governments and will fail.
Syria's 2 million Kurds do not have the money, man power, or infrastructure to become an independent state. They cannot defend against both the Turkish and Syrian militaries. The US will eventually tire of this nation-building task. It is a policy that doesn't make sense.
