I've been a big fan of bootstrap, but I find that these CSS frameworks are becoming more and more irrelevant in the age of widespread flexbox support. CSS modules do put the last nail in the coffin.
I don't find this to be the case. There are a ton of conventions in Bootstrap, an obvious one being its variables, that are very useful when working in a team, and (in our experience) with outside contractors. I couldn't imagine giving up Bootstrap simply 'because flexbox'. Also flexbox support in IE is still terrible, so for those of us building sites for the open-web it's a non-starter for things where a 'just does…
Not to say it isn't frustrating, especially since it works extremely well in Edge but MS said they won't port the fixes over.