Flex is good for creating flows of content, like a <div> where all childs has display:inline-block. It supports wrapping when content can't be on a single line.
Exactly. But we came from using Flex as the defacto system to do layouts and more, now with CSS-Grid it does make sense to use flex for what it was meant for.
I guess it depends on your past experience, but for most of the teams I've been involved in, it was basically a leap from floats to grids for page layout, with flexbox being a small scale option before and after.
Flex is way simpler to use and reason about, it's sufficient for a majority of cases and it works on IE11, which is still supported until 2025.
Grid is more powerful, but not by a huge margin and not in every case. You can get very far with just flex, calc, media queries, and some inline variables and some other tricks. Also grid needs a more global view so to speak and up-front design as opposed to the component based thinking that has become so prevalent.
I'm a fan of the feature but I don't think flex is becoming irrelevant anytime soon.
For page layout sure, but for stuff like a unknown-length sequence of blocks/tiles/cards of mixed size flexbox is getting more useful, especially now grid-gap is just gap and works on flexbox too.
Grid is good for making grids.
Flex is good for creating flows of content, like a <div> where all childs has display:inline-block. It supports wrapping when content can't be on a single line.