The Linux monoculture in internet services does reduce the value of POSIX-compliance.

I got in the habit of at least gesturing towards POSIX back when it was more common to run a bunch of different unixes. And I still run both FreeBSD and Linux at home, so there's value there for me.

Mostly, I still do it because I'm always surprised by which artifacts I create end up being long-lived, and you never know what will be valuable in the future. One of the things I'm technically proudest of ended up only being useful for less than a year, whereas I know a dumb 4 line hack I wrote on a contract in 2002 is still running via cron every night, now on a VM in AWS.