It’s not so much the withdrawal from Syria that bothers many people. It’s the fact that he does things like this on his own without discussing it with the people who know exactly what is going on. He speaks to people like Hannity or listens to people like Limbaugh and Coulter and then makes rash decisions.
In January 2018, the BBC reported that the Taliban are openly active in 70% of the country (being in full control of 14 districts and have an active and open physical presence in a further 263) and that Islamic State is more active in the country than ever before. https://en.wikipedia.org/wiki/War_in_Afghanistan_(2001%E2%80%93present)#2018
ISIS are far from defeated. Why does Trump lie about that?