No matter how much money you've earned, you have to earm more next time?
Yup. Otherwise you stock wouldn't keep rising. You share price is based on your expected profits, if your profits don't meet expectations, you have lost money (reflected by the fall in your stocks value).
Look at Microsoft, for example. They are a doomed business. Once they've finally constructed the ULTIMATE operating system, what's next?
Well that's pretty much the same question Microsoft execs have been asking themselves for the last decade or so. The result of that is the XBox and .Net.
But your example is flawed. First of all, they are never going to make the ultimate OS. Secondly they have a monopoly, so they can just charge more.
In my opinion, Windows XP Professional IS the ultimate OS for me. It does everything I want an OS to do. There is no need for me to invest in anything else (i.e. Vista).
Software support. You might not ugrade to Vista, or whatever comes after that, but eventually the Win32 platform will be phased out, and you will have to upgrade to .Net 2.0 or whatever is powering Windows by then.