[out_of_protocol]

Then you'll need to create own version of migrations, routing, configure asset building pipeline etc etc etc. Phoenix have everything in place, without need to re-implement lots and lots of basics

[Xixi]

A bit pedantic, but migrations are provided by Ecto [1], not by Phoenix. You don't need Phoenix to use Ecto.

https://hexdocs.pm/ecto/Ecto.html

[out_of_protocol]

Well, you'll still need to add Ecto, configure it to use separate configs for prod/dev/test environments etc - so point still stands :)