It's almost like U.2 was created to address these problems...
Yes, but... U.2 doesn't help this much,
unless it's mounted somewhere with sufficient airflow. So, it's still not an adequate substitute for proper thermal throttling by the SSD controller.
I'm with you on U.2, however. I wish consumer motherboards had better support for them.
Bring back the obsolete/dead 1.8" HDD format, and re-use that exclusively for modern SSD's.
No, please don't. Desktop PC cases already have to contend with 5.25", 3.5", and 2.5" form factors. We don't need yet a
4th form factor!
2.5" is small enough for just about any desktop, provides a decent amount of surface area, and ample PCB area that can host a larger number of older, cheaper NAND chips, or go ahead and use newer ones for the largest capacity and best speed.
2.5" HDD format should be left "EXCLUSIVELY" for HDD's.
No, please don't. 2.5" HDDs are dying out. Because HDDs are mostly about having a cost-effective way to add capacity, the 2.5" form factor doesn't make sense. It was mostly an enterprise and laptop thing - enterprise drives used it to enable higher RPMs and shorter stroke distances, which is now pointless because SSDs are
so much faster. Laptops used it to reduce size, weight, and power... but nobody uses HDDs in laptops, anymore.
2.5" HDDs make almost no sense to anyone, any more.