Cadsoft Eagle used a stable and human readable xml format to store schematics and layouts.
KiCad hasn't. It's not even stable (the format).
Define exactly what you mean by "stable", Eagle also introduced new features over the years that obviously made some newer files not fully usable with earlier versions of Eagle.
Now obviously, since Eagle is basically dead and hasn't evolved in years, you can consider the current format fully stable. Sure.
As to the format, KiCad chose S-Expressions rather than XML. Whether you find that less readable is debatable. I'm not a huge fan of this lispy syntax, but that's much easier and faster to parse.
When was the last time you read Eagle schematics and layout XML files and could figure out more things than you would reading a S-Expr KiCad file?
Now having written a library to read Eagle XML files, I can agree that the format (beyond being XML, which was all the rage at the time, a lot less so these days) was well thought-out.