Cracked passenger mirror mount: new mirror

2020 model/2021 edition passenger side mirror got swiped the wrong, non-folding way. Result: cracked plastic mount inside where the mirror assembly seems to be bolted with three bolts to the door bracket. Seemed to me those three bolts could be removed and a new mount stuck on there - looks like it from the exploded diagrams, and you'd think Toyota would build a little fixability into these things these days. BUT dealer says no -- whole mirror assembly must be replaced for ~$600.

What's up with that!