I basically agree with you I think - I don't see highly modular phones as 'the next big thing', or even the thing after that. I just don't think it's presently practicable, and it clearly conflicts with other priorities, like size and weight or environmental sealing.

I'm just pointing out that "desktops are the right size for modularity, smaller things aren't" is more or less totally arbitrary. That's a contingent stage of development we've grown up in, not something based on underlying principles.

In the next few decades tech is going to have to become more ecologically sound (and probably more economical too - to reach the untapped markets). There are several ways of doing that, such as making things fully recyclable. But I think some degree of modularity, like being able to replace processor / RAM / GPU, will probably be part of the solution for phones.