> What do early flexbox implementations (something like 12-15 years ago?)

It shows that a) new technologies may add value and also that browsers constantly fix bugs due to complex or underspecified standards. It was just an example, go through the issues lists and you find a lot more.

> not the ones who jump to use new browser features too quickly

That's reasonable but there is a point where support just should be dropped. Don't support IE 5 for ages just because someone is too scared to update. Let them fix their technical debt. Encouraging people to access the internet with insecure software is not acceptable. Btw. that was the argument you completely ignored in your reply.