> xz wasn't chosen for its archival integrity AFAIK, it was chosen for its size.
No: that's why the LZMA compression algorithm is being used. The switch from the original LZMA container format to the "improved" xz container format had nothing to do with size, as the old container generated slightly smaller files in addition to not having this combination of negative properties.
I remember the hype around LZMA2 and xz being that, because it uses blocks, you can do the compression and decompression on multiple threads. LZMA was generally considered too slow to use for regular usage (only for archival, basically) and multicore was the new way to scale so this was perfect.