You absolutely can produce an open source smartphone. Pine64 has done it. ubuntu and purism have done it. Nokia was doing it, woot n900. Many chinese brands do it. fairphone kinda?
They can build fully open hardware and we have loads of OS that work just fine.
But why then does no mobility provider at all offer them in their phone offerings. Even if they were on page 34. Tmobile usa, everyone in canada for sure, has AOSP and KaiOS for dumb flip phones. But no open options? If you bring an open phone to them, you'll be imei capped and isolated and separately tracked.
But the answer comes when you also realize ulefone or umidigi are also not offered. It's about who is in control. I highly recommend your next phone is imported from a different region than you are in.