I feel like there is some context missing in your story here. There is a lot of middleground between heavy SPA frameworks and creating everything from scratch. More importantly, I am left wondering what sort of functionality was your team trying to build that requires that much interactivity? At least that is what I assume with "rich user experience"?
If I had to guess, they were probably wanting to implement something like animations into the UI. Animating a list of items onload in a staggered format is still basically impossible unless 100% of your users are using Chrome. With a JS animation+component library, this type of animation is pretty much plug-n-play.
When the startup is trying to attract customers and also impress investors, sometimes there is a lot of effort spent on the investors just so they keep putting money into the machine. "See! We have an ultra modern/sleek site so it must be some other variable that is causing customers to churn..."
