The business requirements to the architecture — not functionality! — change over time. At first, you have to be lean as hell and try a lot of different stuff. Then, you need to scale, and performance becomes an issue. Some time afterwards, you want to add more features to increase monetization and retention, and here you start needing good, abstracted out architecture for all this stuff not to interfere with each other. And finally, you find the sweet spot, and start to really care about bug-free experience, maintainability and reliability.

And all these different business priorities are best suited by different languages, different design approaches, different architectures and people with different personality traits.