I'm sympathetic to the desire for an "install and forget" drop-in Vim replacement, but...don't you think that this runs contrary to the purpose of Vim/NeoVim as a flexible, customizable editor? If you're an advanced enough user to have a nontrivial vimrc, then it's entirely possible that you'd also want different configurations for vim vs nvim, and that you'd want to be able to switch between them easily if you discover something doesn't work in nvim (especially since nvim is not yet at version 1.0). It's also probable that a lot of Vim users wish that more classic Unix/POSIX tools followed XDG, rather than requiring rc files in your home directory. As for Bash, not everyone uses it, there's no reliable way to automatically insert content into a bashrc file without potentially screwing things up, and Windows doesn't even have a reliable way to run a Bash script (assuming some version of Bash is even installed).

I do think it would be reasonable for the neovim installer (on all systems) to have an option to create an init.vim file that reads your vimrc, and possibly even to create a shell alias as you describe. But these should definitely be opt-in, not opt-out.