Not necessarily. I’m sure Chris is strongly motivated to not to cause any undue effort for pre-1.4 adopters but … essentially using Brunch with 1.4 (and above) is a similar position to using pre-1.4 Phoenix with anything but Brunch. The default JS tools aren’t really a core concern of Phoenix - they are merely supplied as a convenience.
If this is a Babel issue one has to remember that Babel does not target language specifications but rather feature sets as they are provided by various browsers and Node. UMD modules are starting to fall out of favour and the code therein is largely becoming ES2015.
How to write and build JS libraries in 2018
It’s basically becoming more and more difficult to pre-compile all the various permutations that people may want, so transpiling of vendor code may become a user responsibility again.
So when you are finally ready to leave Brunch behind you may not want to blindly adopt webpack 4 but spend some time to find something that suits you - because more than likely you’ll be spending some significant time with it.
