Something that I think not everybody realizes is that Flask does not require extensions for most things. The extensions are nice because they integrate with the Flask configuration object, and with before/after request handlers when it makes sense, but none of that is required. If an extension stops being maintained, you can always find a newer general purpose Python package that does what you need and just use it in your Flask app.

Now this is my personal opinion, but you cannot say the same about FastAPI. Even though the ecosystem of asyncio compatible libraries has grown, it is still several orders of magnitude smaller than what is available for standard non-async Python. So in my view, in terms of access to 3rd party libraries Flask is a better choice.