IMO, this can be simplified to the fact, as mentioned by others, you don't own the seat assignment.

Delta can (without your permission) move you or they can't.

Obviously we can all argue about what we want, or what they should do, but at the end of the day it doesn't matter. You can't argue with "but I want..." if Delta, by way of contract/agreement have the right to move you without your permission.

Ultimately you make your choice by the airline you choose to fly. Don't like it try another airline.