I like the idea in principle. I do think it's really useful to customize a few specific parts - one person might want a high-performance (and large, and expensive)) camera module both front anb back; another prefers just a minimal camera and gets a larger battery instead; a third has a job where cameras are banned and opts to get none at all. A fingerprint reader, a headphone jack, or an SD card slot are other options people may want to add or skip.
But I do not think upgradeable phones are meaningful. After 2-3 years with a phone, it's pretty beat up. Screen is scratched and dimming, the case is scuffed and creaky, buttons don't quite work, connectors are getting glitchy, the battery is dying and both CPU and memory are getting old. I'd want to upgrade all of it - I want a new phone, not throw money at the old one.