Why would you need to migrate to Python later? I can understand needing to migrate to something more performant/easy to scale, but Python doesn't have any advantage on Ruby in that department.
You still didn't answer why would I want to migrate away from Rails. Python has no advantage I can think of (unless one has to work with data analysis).
My experience is admittedly tainted because we migrated away from a Rails codebase and it was not fun. The Rails code was extremely convoluted and encourages metaprogramming in a way that is very un-typed.
Basically, I think Rails is inferior because it encourages bad practices, has no distinct advantages over Python, Python seems to have a larger talent pool, and Python has uses outside of Rails that make it a better choice for a one-language shop (i.e., your tools can be in the same language as your code and can utilize that + Python is everywhere, Ruby has extra devops costs incurred).
I won't say you can't be successful with Ruby. That'd be silly. But do I think there is a reason to choose Rails over Python if you can start fresh? Nope.