Same thing, you will want to reuse existing code, and you do not want to split knowledge in your student group. There is no such thing as 'gradual' migration: I either have to support Python in my group, or Python and Julia -- until everything is migrated.

If I have a working ecosystem using Python, with students trained in Python, and all previous work in Python, there's a whole lot of opportunity cost associated with me deciding to have the next student use Julia. I'd rather have that student build on existing tools and knowledge and do something new with their time.