True, this puts the onus on the CTO not the junior developer who will make lots of dumb choices, not just shiny-chasing.
But there's no reason you can't work a day-job doing boring technology and do fun stuff on your side projects.
Companies should actively support your side-projects, especially if they involve R&D experimentation that could be useful in the long run to the company. Whether that's paying for you to attend educational events or any books/online video courses/supporting SaaS services/etc or giving you 10-20% of your week to do what you want.
There are too many CTOs and senior developers out there who are too busy working on their pet projects to care.
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.
Why would anyone pay a senior developer $100-200k+ to sit in a 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.
But there's no reason you can't work a day-job doing boring technology and do fun stuff on your side projects.
Companies should actively support your side-projects, especially if they involve R&D experimentation that could be useful in the long run to the company. Whether that's paying for you to attend educational events or any books/online video courses/supporting SaaS services/etc or giving you 10-20% of your week to do what you want.