In many ways you’re correct, the “modern” js toolkit can be a nightmare. I work for a SaaS company that makes emergency management software. I’m pretty proud of our setup. It’s Vue 3, it’s incredibly optimized. We have tree shaking and code splitting, e.g. there’s hundreds of potential JS files you might load using our software, but they will only load when you need them, over a brotli compressed HTTP/2 connection so it really is efficient.
With the amount of data we process through our API and how it’s presented to the client it would be a nightmare to not have Vue for state management and routing, axios for API calls, etc. But many SaaS products certainly aren’t optimized like ours