Your question is irrelevant. If you don't care about security, SHA1 is a bad choice because there are faster hash functions out there. If you do care about security, SHA1 is a bad choice because it has known flaws and there exist other algorithms that don't. The only valid reason to use SHA1 is if there is a historical requirement to use it that you can't reasonably change.

Any analysis about how hard it is for an attacker to get a file on your local file system via a cloned got repo, cached file, email attachment, image download, shared drive, etc is just a distraction.

You would be right, except that there are no faster hash functions (on all modern Intel, AMD and ARM CPUs, which have hardware SHA-1).

BLAKE 3 is faster only in wall clock time, on an otherwise idle computer, because it fully uses all CPU cores, but it does not do less work.

BLAKE 3 is preferable only when the computer does nothing else but hashing.

Uh no.

On a modern intel CPU, one core of SHA1 does about 500MB/s worth of hashing. Blake3 on the same core is 1.5GB/s or faster.

On the ThinkPad I'm typing this on, the single-threaded BLAKE3 benchmarks hit 8.8 GB/s :)