Since the topic is "new framework", I would not advise investing in a system that is not ideal long term just because of a very minor support percent difference today. (looks like 6% of the world based on caniuse.com, but your audience may vary of course)
The benefit of an assumption like "everything by default is vertical", then the few people without grid support, but with flex, will have a "mostly" ok display.
Also, it's not hard to have a fallback to flex for horizontal items. (the only place it would really matter)
I feel like this would actually be one of the strengths of using a framework. I can abstract away my own styling to the framework, and the framework can upgrade from flexbox to grid without requiring me to change any of my own code.
Yep, I agree, seems like we are on the same page. I made my own framework based on my specific uses, so I even get to control when the fallbacks are removed, and deal with any of the fallout if there's issues.
I kind of think that at one point, everyone that works a lot with front end web development should make their own framework. It helps you learn to make long term decisions, deal with design iterations where you have lots of wide spread use of older systems and many other things that help you make wiser decisions in day to day interface design. But also it can help you see where things like Bootstrap are helpful and where it is not.
The benefit of an assumption like "everything by default is vertical", then the few people without grid support, but with flex, will have a "mostly" ok display.
Also, it's not hard to have a fallback to flex for horizontal items. (the only place it would really matter)