Microsoft seems like one of the more reliable partners to build on compared to Google etc. just for the simple reason that their customers are large businesses and not breaking things for them is in their blood. Just like Windows backwards compatibility.
I actually wouldn't be worried in the short term for exactly this reason. Microsoft has legal access to GPT-4 and is allowed to host and serve it via Azure. If OpenAI somehow tanks its API in the near term, MS is sitting on a gold mine and will make use of that by continuing to serve it. In the long term I am worried, but less worried if Sam and Greg form a competing co to continue to build.