Sure. But if the choice is NixOS alone or NixOS hosted by Bedrock, then NixOS has a pile of advantages which Bedrock would nullify, chief among them being the ability to atomically upgrade and rollback the entire userland, service configuration, and kernel. These are non-trivial goals of NixOS.
It is not clear to me why you feel this way. Everyone I've spoken with who has put effort toward making Bedrock play nicely NixOS has done so with the aim of retaining that list of NixOS's strengths in the NixOS slice of the system. Let Bedrock proper fade to the background: for a user who is NixOS-oriented, such a system could be NixOS, with all of NixOS's strengths, with the ability to add or swap out (non-atomic upgrade/rollback) parts from other distros.
I feel this way chiefly because I actually do use the features of NixOS to atomically rollback entire machines in production, and to atomically upgrade my development workstation. Having to manage an additional piece which is beyond Nix, whether that's EFI configuration or virtualization or Bedrock, is acceptable but very irritating.
Apologies, I misinterpreted your concern to be a matter of losing the entirety of Nix's atomic features entirely rather than concern over even limited escapes from its Nix's scope. In this case it does sound like Bedrock's trade-offs won't be worthwhile for you even in the hypothetical future where it plays nicely with NixOS but retains the non-atomic nature of components from other distros.
