The foremost role of senior devs (at least in a company with juniors and mid-levels) should be technical oversight. Set up the architecture, set standards, and do the code reviews.
Not sit in the corner and learn React.js.
I was talking about junior devs making (predictable) bad choices not senior devs. Your senior developer should know better than to shiny chase and value delivery over cute technology, otherwise he's not a senior developer.
Some startups can't afford a senior developer or simply don't know any better... but even junior can pump out working apps. Then it's just up to the next evolution round when you build a professional team instead of some cowboys.