To me it seems like your examples kind of prove the opposite of what you're saying. Prior to October 2001, The Taliban controlled a small and important part of Afghanistan—but so does the government in Afghanistan that we recognize now. In both cases it was/is still considered a legitimate state. If the U.S. military were to pull out completely, Ghani would control Kabul and that's it. Yet we and most of the international community would
still recognize Afghanistan as a state.
Likewise, Syria and Iraq are considered states, but they obviously don't control the territory they claim. But for the most part, IS does:

To be considered a state, the general requirements in IR 101 are a people, territory, and a monopoly on violence/ability to defend that territory. To me, IS hits those check boxes—certainly more so than a lot of things that we consider states (such as your examples.) The last requirement is recognition from the international community. I think that is just a matter of time. I do not see the Sykes-Picot borders carrying on through the 21st century.
I'd like to take a shot at The Walrus's questions because I don't agree with some of the answers here, but I'll have to come back to that later.