My colleague @fpletz wrote a detailed explanation of the decision in Undeprecate networking.useDHCP · Issue #75515 · NixOS/nixpkgs · GitHub. However there are a few people against this, so feel free to contribute to the discussion.
Btw, nixos-generate-config can take care of this for you since nixos-generate-config: add useDHCP per interface · NixOS/nixpkgs@5ee383e · GitHub.