My 2 cents on that is:
Most users do not start projects which needs so many customisations from day one (and if they do, then I would say, they do something wrong). In a lot of cases people will do mix phx.new once, and then they will go with the project, because let’s be honest, it is not that often that you start new projects from the ground up. For sure it is less often than adding features to existing projects.
For example, I am using completely different approach for naming my controllers and stuff (instead of MyAppWeb.FooController I use MyAppWeb.Controllers.Foo to follow naming convention used almost all other Elixir projects) which requires me to do “few” manual steps. I do it by the hand each time, and TBH I never had a problem with that, because in last year I needed to do it twice. Oh, and by the way, I am not using Webpack at all, so I need to change it to Parcel as well, so this is quite some time for preparing that. Having such generator maybe would be useful, but I do all of that so rarely, that xkcd: Automation and xkcd: Is It Worth the Time? describes why it is not worth my time.
