Valve is "de facto" monopoly, bit the actual monopoly potential is in Microsoft hands. Microsoft is for PC gaming industry what Google is for the web browser one. Sure, there may be other cool web browsers, but it's Google that (through Android base) decide whic web browser will be delivered with the next billions of Android mobile device: some elderly people on smartphone don't even know what is a web browser ("oh, you mean when I Google? I don't know: I just Google").
All future new PC will be sold with Microsoft Store and Xbox junk ware: Microsoft has been exceptionally shitty for not being the actual monopoly in the PC gaming industry. But that's a very feeble protection: break Valve business is just a mandatory "security update" away to happen. They can break Steam little by little (such as suggested by Tim Sweeney) or just a big blow by sheer monopolized manipulation (such as Google not allowing adblockers to chrome to feed their advertising business)