Easier said than done. Phones are tiny, so you can’t really fit everything in a modular grid system since things need to overlap, stack, etc… And you can’t just make everything the same size every year because things just physically take up different amounts of room as they change and there isn’t any room to spare for future upgradability. Batteries grow/shrink, cameras get turned into sideways periscopic lenses, fingerprint sensors grow and get placed under screens, etc. You can’t just tack on an sd card reader or ir blaster if there’s no space for it. Not to mention cpus change and you can’t just run everything over pcie and ribbon cables.

Realistically, what would even be upgradable in a phone that wouldn’t require resizing or repositioning internal components?

Battery? We can already do that but manufacturers refuse to in favor of sub-par water resistance and planed obscelesce. Plus, battery tech doesn’t advance quickly enough that you’d get a meaningful capacity boost in the same footprint.

Cpu or ram? Upgrades really won’t make a meaningful difference until your entire phone is old enough that it makes more sense to buy a new one.

Camera? Maybe slightly more useful than upgrading cpu/ram, but cameras are expensive and heavily depend on economies of scale to be as good as they are without costing a fortune.

Also, it’s really easy to break your phone when you open it up, so there’d need to be additional barriers and hardened connectors to make it user friendly, which again takes up precious space.

Just make the phone a brick, you say? That might work…

OS replacement, however, is easy. Just unlock the bootloader, no sd slot needed. Any phone could do it.