Once you get to a certain stage, yes, modularity becomes very expensive. I do believe that things like SSDs or WiFi cards should be modular, because there is no reason for them not to, but RAM? We are moving towards high-bandwidth on-package RAM, why would I want to sacrifice 100Gbps of performance and 5+ hours of battery life — not to mention also paying more — just to have an option of upgrading the RAM some time down the line?

Right, and then even if you can upgrade the RAM, the machine may not even be compatible with the best RAM down the line anyway. It's hard to imagine another drop-in upgrade that will be as big as putting SSDs in old machines was.