okay, the main point in what you are saying is true.

if a company is working in a market that is reaching maturity or decline, it should look for a new market where it can invest its effort

but the trick is simple, MS have certain competencies (skills) the new market must be a market where its competencies would be relevant ... the even trickier part is, the relevence must not be so obvious, the product can be completly different, but the needed skills could be same

for example, Dell is good at complex assemblies, so maybe dell, can start assembling cars, instead of PCs, because a car can be customized in ways comparable to a PC

of course this is just an example, but you get the idea