If Debian is perfectly fine with anti-POSIX, why go to the effort of making and switching to dash over bash? One of the impetuses for that project was bash’s inability to confirm to posix even when launched in posix mode. What’s that about?
As I understand it, the main motivations were speed and memory footprint. Might not matter too much on a desktop, but people also use Debian for smaller things like Freedombox.
Ubuntu made the switch to dash back in 2006 to decrease boot times, as dash is much faster than bash. This was upstreamed to Debian. POSIX compatibility made the transition easier (as most scripts were made to run on many Unix systems, not just Linux), but was not the motivation for the switch.
Well Debian isn't some BDFL's project, different authors have different visions that sometimes don't agree with each other. With dash, the original advantages put forward were minimalism, higher speed, and POSIX compliance [1]. But that was a long time ago and when systemd came, the preference for dash may have changed for many Debian authors and maybe dash is just kept in Debian as a token of the past accomplishments.