FastMail is using JMAP, a protocol designed to support push notifications. MXRoute is stuck with SMTP/IMAP which don't support those. There might be a different reason, but I think this is probably the core issue here
This is an issue with nixpkgs not nix. Google could've just bootstrapped their own nixpkgs from scratch if they wanted to, see Guix (not a perfect example but still). Creating a whole new tool is still completely unnecessary
nixpkgs already has 107158 packaged libraries/executables. Nix has infrastructure to support arbitrary build systems and can create docker images. I fail to see any advantages of creating a more narrow version of it that has fewer uses and has to start from scratch