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>> sure maybe you have some data science people who need python,
This is how it happens though; it's not "let's form a company with 10 developers; don't worry what tools they use!". It's starting with a single problem using common tools, then adding specialized problems where you could still use the same tools but they are not optimized, then adding an acquisition product that uses different tech, then growing to 100 or 1000 developers and may all use React or C# (doubtful) but don't use it the same way...
>> If you're going to make a change then migrate everything
Have you ever worked for a software company before? THis is not how it goes.