IBM is still with us. They're having to reinvent themselves regularly, because the only concept they own is still "Mainframe", and they failed to hold on to "PC". But they're still a huge company selling systems and professional services.
I expect Microsoft to go the same way. There will be significant but dwindling demand for Windows for 30 more years. (But probably not Azure, Windows Mobile, or XBox.) They will stay alive, and even prosper moderately, on that business.