True - and you can upgrade the sd card in your phone (which is the equivalent of the hard drive). I agree, making memory upgradable should be doable, but it took quite a while for laptops to start doing it as well.
I suspect that the costs won't work out. Even with laptops nowadays it's rarely worth upgrading the memory after purchase - by that time a new laptop is not much more expensive and far more powerful. Having the option for more memory initially would certainly be an improvement though!
I think you ought to talk to a skilled embedded device hardware engineer first. It certainly sounds a reasonable idea but I suspect the situation's far more complicated than it looks to a layman.