Response #1:
Yeah man, it's all capitalism, they don't do it out of the goodness of their hearts!
What a revelation!
Pass the bong.
Response #2:
They're is no "Microsoft" as a person. That's a company, an aggregate entity which doesn't have either brains or a strategy.
What DOES exist is people running the company. Those people can change (and have, Gates->Ballmer->this new guy), and these new people can have different ideas about how to go about things.
If there are any propritary companies you like, then imagine that MS could be turned to behave like them. There is no "evil brain" behind it all making sure MS will always be like in the 1998 carricature you have in mind.
IBM circa 1999-2004 was hailed by OSS advocates for supporting Linux and OS -- yet the same company was the fearful Microsoft equivalent of its era, a few decades back.
Oh, and this evil AT&T that everybody feared and they had to break up in an anti-trust case? This is the company that gave us UNIX in the first place. (Not to mention Xerox Parc, another company's proprietary research facility, without which we'd have shit today).
And vice versa: those "nice" companies could change course if the wrong people came on board, or their current people saw an opportunity and got greedy.
Response #3:
A lot of us don't give a flying fuck about the success of OSS as some ideological venture.
We like some OSS software, and we like some proprietary software. Heck, some of us make a living creating proprietary software for proprietary companies, from small indie firms to Adobe to Microsoft to what have you.
And even as users, a lot of us won't settle for an inferior OSS product is there's an (affordable) superior proprietary one (and vice versa).