To address the other points you raised; I personally found Babel 6 easier to use and understand than Babel 5. I suspect this is because I needed to configure Babel 5 anyway to meet my needs, so I was required to understand the configuration either way. Babel 6 made it much easier to understand what I was asking Babel to do; no magic, everything explicit.

With version 5, the default settings were a moving target (which I found confusing), and explicitly configuring it was more complex. Apparently the Babel team had great difficultly deciding what should be included by default as well.

Basically anybody that used Babel for something other than "6to5" had to jump through hoops to overwrite the defaults. Seeing as how the Babel team were trying to position it as a general purpose Javascript compiler, that was a problem for them.

So, maybe it's worse off for most people because of the change (because it doesn't work with zero configuration anymore), but plenty of people were better off because of it. The library became easier to maintain, and easier to use for a non-negligible number of users.