There's lots of things Microsoft can do to make Windows more valuable and thus charge more for it, and additional services they can sell. They didn't just sell

There's lots of things Microsoft can do to make Windows more valuable and thus charge more for it, and additional services they can sell. They didn't just sell Windows but also a ton of software on top (Encarta, Flight Simulator), added game consoles, made the OS more stable etc. But that doesn't mean a single product line will continue to grow vs. stagnate. You only get to invent the word processor once.