Your skepticism about the viability of enterprise-level applications built without heavy frontend frameworks reveals a narrow understanding of software history and architecture. Let's not forget that companies like Amazon, Google, and eBay built scalable, complex systems long before React or Angular were even conceived. Amazon's initial architecture was famously built on Perl, and Google's original frontend was simple HTML and JavaScript. eBay was initially built using Java Servlets and JSP, and it scaled to handle millions of transactions. These are not 'imaginary success stories'; they are well-documented cases that prove scalability is not solely the domain of modern frontend frameworks.
Your assertion that older frameworks 'became a mess at scale' is a sweeping generalization. Any technology can become a mess if not properly managed; that's not a failing of the technology but of the architecture and development practices. PHP, for instance, powers Wikipedia, one of the top-ten most visited websites in the world, and it handles this scale quite well.
The emergence of frontend frameworks was not solely because 'existing solutions did not scale'; it was also driven by the industry's evolving needs for richer user experiences and faster development cycles. These frameworks solve specific problems but come with their own trade-offs, including complexity and steep learning curves. To dismiss alternative approaches as unscalable without evidence is not just intellectually lazy; it's a disservice to the field of software engineering, which thrives on diverse solutions to complex problems.