Essential is defined as the minimal set of functionality that must be available and usable on the system at all times, even when packages are in the “Unpacked” state. Packages are tagged essential for a system using the Essential control field. The format of the Essential control field is described in Essential.

https://www.debian.org/doc/debian-policy/#essential-packages

Diversions are addressed in 3.9: https://www.debian.org/doc/debian-policy/#maintainer-scripts

Debian Policy, generally, tells package maintainers, not users, what they may or may not do. Several classes of conflict are listed in the document; Microsoft's scrips violate multiple requirements.

The goal and result of this ststem, arrived at over 21 years collaborative development (Policy was first published in August 1996) is a system which provides well over 60,000 packages and behaves surprisingly robustly from the user-administrator's perspective. Which is to say, there are few surprises.

Microsoft have violated multiple Policy requirements. This is considered a release-critical bug in distro packages for very good reasons.