What would this do, though? You'd deploy the heaviest lance you can so you have the best chance to brawl through the mission, and you'd withdraw whenever you want.
Well I did say early to mid game because late game assault glut is a complicated issue stemming from limited lance space and restrictive mission parameters.
What's stopping players from always picking their heaviest mechs? Not if the objective is always to kill the opfor. But, if the mission is designed in such way that favors mobility and discourage combat (extremely heavy opfor and 0 salvage opportunity for example) then players can choose to either avoid combat to play the objective or try to kill everything anyway but for no extra gain.
If there are enough such missions, especially with hidden changes, then the players might favor a more balanced lance as opposed to a kill team setup.
@Prussian Havoc I don't see a retreating AI as being impossible. The panic mod already showed us how AI can be weighted to abandon combat when all hope seems lost. Instead of pulling the ejection lever and giving us all the loot, a code to tell it to sprint for the nearest map edge doesn't seem to be impossible. It can even work both ways so your merc, given certain trait, will stay till the bitter end or ignore your command and escape themselves.