Developing Microsoft desktop software you're still building on someone else's platform and hoping they don't decide to alter the terms of the deal. Microsoft may be more forward thinking than Google, but companies change.
Proprietary platforms lure developers to their stack by making development easy. Learning an open source is typically more difficult, but the reward is greater freedom. Believing you're forced to build on proprietary technology is a fallacy.
And you will always be relying on someone else's proprietary tech - whether it's laptops or power stations or cloud infra. The trick is deciding then to outsource and when to build your own.
It's in the long term interest of any business not to rely on the goodwill of some other business. This goes for Microsoft as much as for Google, Apple, Amazon, etc.