For me, as a founder of a young startup, the overarching benefit of being proficient in modern JS and sorrounding tooling is that I can use the same language for my whole stack(servers, web, Android, iOS), and spend more time thinking about the product and business. In the possible future where the company grows enough where it needs a distributed server software, the only part of the product that'd be changing would be the backend, and I'd probably look at Java for that.
For the rest, React(& react-native) + graphql + a typesystem is a pretty good stack for UI software.
