I have extensively benchmarked Zstd and it is a joke compared to LZMA2 when it comes to compression ratio. And not even that, the lack of features Zstd has, that 7Z does have, makes it a far bigger joke. 7Z is a feature complete archival solution unlike Zstd, with possible options for archive repair. RAR is far superior for that bitrot resistance.

The amount of possibilities Facebook and US gov get with backdooring XZ are endless, since it could destroy trust in it if uncaught, and Zstd adoption meant web malware deployment could become a matter of when, because Facebook already does it right now with actual malware JS scripts through fbcdn domain.