there are a number of things going on in the general computing market that you touch upon here.
generally speaking, hardware is not very profitable. software, on the other hand, is usually bonkers money making - so long as people are prepared to pay. [microtransactions is a model that gets people paying many times the 'value' or 'cost' of the software they are using (Candy Crush made billions).]
there is a strong trend towards online subscription models for software. combined with 'cloud' delivery (eg Office 365) and cloud storage, this ensures the publisher captures as much income as possible from their software-as-a-service.
most people do not upgrade hardware, they replace it. it is still possible to buy components and to buy custom boxes, but most end-users treat computers like any other consumable: more like their car than their games - they replace the whole thing rather than fiddle about with whatever-the-hell is inside.
building devices that are not readily upgradeable reduces build costs, and reduces secondary inventory - you don't have to carry as many replacement parts, nor hold as much stock-on-hand, if your products are replaceable rather than rebuildable. you can organise customer service around just-in-time or only-when-ordered parts rather than holding racks of parts on hand to cover repairs and cover swap-outs.
most users just want their devices to work. they don't really care about the hardware or software (in the ways that 'techies' do) - they just want the thing to work, and to not have to fight with it all the time. they might care about having the 'best' screen they can get, or whatever, but people tend to prefer to 'stick with what they know' (which is part of how platform capture works/happens). (there is a reason people who obsess about having the 'best' components or are fixated on specific platforms are considered to be 'hardcore' or 'fanboys' - they're outliers from the norm.)
we can, and hopefully will continue to be able to, build custom boxes and swap out components of hw and sw for some time to come. but most people treat computers like cars: they want to be able to hop in and drive.
so long as there are many vendors catering to them, there will be a fight for their attention, and to offer something that is 'good enough' at a price people are willing/able to afford.