[flimsypremise]

Not sure what you're really saying here. Teams often use languages because their existing codebases and the proficiencies of their existing members, but "we use that language for backend so we should use it for frontend" is totally a thing. You're somehow seeing multiple languages as an advantage when its actually a form of debt. Teams work in multiple languages because prior choices force them to, but if you can limit the number of languages and technologies in use it allows you to streamline and automate spinning up new services, make it easier to hire, and greatly simplifies deployment configuration. This is one of the major things that happen as you transition from an early stage to late stage as a startup. You hire professionals after a few years of not knowing what you're doing, and they cull all of the non-standard shit, lock in your architecture and languages, and then template our your apps, your configuration and your deployments.