On-package RAM and upgradability are not at odds with eachother. If upgradability was desired, we could see socketed SOC. This is one of the things modular phones (project Ara) were about.

Just because RAM was one of the last holdout of upgradability, does not make it inherently more suitable for upgradability.

The problem is a lack of interest in manufacturing repairable and upgradable hardware. It is simply less profitable.