@oCrapaCreeper: Thanks a lot, that finally sorts it out for me.
TL;DR: They are redoing the launcher in C++ for various technical reasons. That project has been in place for 4+ years, so it is not MS related. Main advantages: Works around issues with Java installations and various platform-related bugs, allows management of Java on OSes without central package management (Win/OSX), and allows Mojang to host non-Java games, like C++ Minecraft. Downsides: None really.
@The_MP8: Not a normal update. This is about a completely rewritten beta launcher that is not available by official channels yet.