This can be done with traditional nix commands too, using the nixos-config component of NIX_PATH.
Less intuitive and somewhat akwkward, sure, but it’s possible.
Generally flakes just have much nicer UX. Channels are indeed gone completely, and with them the issues caused by having multiple and not understanding the implications on updates.
There are some downsides too, like making it more awkward to match up different projects into one version of nixpkgs (for disk space reasons). Some others and general arguments live in this thread.
Personally I think the upsides by far overshadow the issues e.g. with installing through nix-env (nix-env -qaP is quite ok still), and that the risk of potential breakage for NixOS configuration is very minimal.
Either way, the point is, nix is stable and has been for years, flakes are not required, but very nice, especially for newish users.