Although, ultimately, this feels like a market/advertising issue over a technical issue. While I'm not convinced a 100% recycle phone will ever be a likelihood; surely we can get to 90%. Or slow down the trash generation with the aforementioned modular phone.
However it's going to require people to want to spend money on such a thing and also industry to want to produce and advertise such a thing. None of which seems like it's happening anytime soon.