The only thing I will say is that I encourage you to get involved with hardware engineering. You will quickly find that small personal electronics have extreme design constraints that are quite unlike a custom-built PC (it's like complaining that you can't just buy RAM, a CPU, enclosure, etc and build your own ECM for your car).
People don't drop their battery-powered custom-built PC into 6ft of water and expect it to keep working (then dunk it again after two years of abuse, 4 floor drops, etc have worked against the case, seals, and so forth). They don't take it from freezing temperatures into the warm indoors and expect it to keep on trucking. They don't expose it to extreme temperatures on car dashboards in the summertime and expect it to still perform (it would absolutely hard-lock due to overheating if you tried it). Compared to a phone it doesn't matter very much how much a custom-built PC weighs +/- 1kg; phones fight for grams. If a custom-built PC uses an extra 15w who cares? But that might be more than the entire power budget of a phone SoC. People expect a phone not to spew EM that breaks the ability of anyone around them to use data or make calls. People also expect their phone to be able to complete a 911 call in an emergency so long as some kind of signal exists.
Modularity IS NOT FREE. STOP ACTING LIKE IT IS.
Modularity costs space, weight, and complexity (which often translates into user time spent troubleshooting).
If a user-replaceable screen means giving up waterproofing do you expect that to be a popular tradeoff? If making the battery replaceable reduces battery life by 40% is that a good tradeoff?
It is clear to me some people complaining haven't spent any time researching this topic and have no idea just how much engineering goes into modern electronics nor what the tradeoffs are. If they actually had to live with the results of their claimed preferences a lot of them would hate it and switch back immediately. At best I see people hand-waving half the battery life or double the weight as if it such things were trivial for devices people hold or carry on their person for hours a day.
I'm 100% serious when I say if you are working on your own company or product please make sure you approach these things with eyes open. If you are deliberately going to serve a different part of the market know that going in. It's fine to go after a niche - a niche can be profitable - but understand your customers and what they really value (not just what they claim to value). Don't let a bunch of contrarians on HN convince you there's a market for 10 million modular cell phones. You'll lose a lot of money when your "customers" skewer your product for all the compromises necessary to give them what they claimed to want.