Its not for nothing that Debian is called the Universal operating system. Last I checked, laptops on the International Space Station weren't said to be running Android, or even AOSP. They were in fact running Debian.

> has permissions and isolation for apps which Debian doesn't have

"Debian buster has AppArmor enabled per default. AppArmor is a mandatory access control framework for restricting programs' capabilities (such as mount, ptrace, and signal permissions, or file read, write, and execute access) by defining per-program profiles. ..."

> Why don't they want to use at least Dalvik...

Dalvik is dead anyway. Modern AOSP versions use ART.