"No-brainer" depends on who you ask. It seems like a good idea and the tech seems solid... but Zstandard is made by a guy at Facebook and seems to be owned by Facebook as well, since it's hosted on their github: https://github.com/facebook/zstd

On the other hand, according to https://facebook.github.io/zstd it seems that zstd is part of the Linux kernel now, so if you're all-in with Linux then it can't really be any worse. Personally the Facebook angle doesn't bother me all that much because privacy has been completely lost now (and anyone who honestly thinks they can defeat the might of all these corporations and governments is deluding themselves), so I'm all for the change. The code is open after all.

The increase in package size would worry me. The download and storage size of packages is nearly always more important than the decompression speed, since the package can be decompressed in parallel with the download.

Surely xz could be modified to produce the same bitstream on single core machines?