Learning a language like Python is easy enough, but the hard bit is actually unlearning the language you came from. If you give a JS dev a couple of weeks to learn Python they'll probably succeed, but their code will be 'python syntax written with a JS thought process'. The code will work but it won't be good code. Sometimes that's fine, but as the foundations of a startup?

Getting to the point where you're writing good Python and taking advantage of what the language provides, knowing the ecosystem, and building things that other Python devs won't look at and think "Wtf?" takes much longer than weeks.