I feel like that could make some forms of centralization pop up. If you have all these currency notes, who's making them? Someone's gotta manufacture and make those notes and coins. Also, I think the strong benefits of using bitcoin come from the fact that you have a lot of flexibility and precision in how much money you want to send to someone. If we make some kind of notes for BTC, you lose out on that pretty nice benefit. Genuinely, we as a society are likely going to be moving away from currencies that are physical and move towards the electronics based ones. They're just a lot more fitting when it comes to the future and potential uses for crypto.
This is the most important point that has been brought up in this thread. Who would make paper notes or metal coins for BTC? How would you avoid counterfeits? Why would you even actually want paper currency? Everything is going electronic anyway. I can't remember the last time I used cash. I just use Google Pay on my phone to pay for everything. If that's the case with fiat, then it most definitely must be the case with crypto. Making some transactions with paper BTC would completely defeat the purpose of the blockchain.