Those won't remain PCs forever. "Post-PC" devices are already making substantial inroads, just to a much smaller degree so far. How many business laptops meant for field use have been replaced with smartphones and tablets because all they're used for is reading e-mails and viewing maps and such?
There's no doubt that MS can coast for a long time on PCs, but if they do, they're never going to dominate they way they did in the 90s, and they will eventually go away.
You are absolutely right. But I'm pretty sure (crystal ball time) the post-PC device looks a good deal closer to a Surface Pro than an iPad.
That concept of keyboard + mouse + touch screen is really good; you need a way to have custom line of business apps developed for it. Apps need to be easily installable on it. It needs to have integration capabilities with oddish hardware. App state needs to be 'sane' and persist extant data without wrecking it due to swapping out of memory.
IOW, the capability lineup looks a lot like a PC on someone's desk, even though the form factor might reflow to be a tablet placed in a dock with attached bluetooth keyboard/mouse.
You could be totally right about that. It's a bit of a tough sell to me to say that Surface will keep them relevant, but it's definitely far more possible and reasonable than them staying relevant sticking to the PC.
There's no doubt that MS can coast for a long time on PCs, but if they do, they're never going to dominate they way they did in the 90s, and they will eventually go away.