Lower fidelity would also mean to restrict tracking. Good luck with that. Internet advertising and data mining companies are not exactly renowned for their moral and ethical integrity. They would have no issue draining the data plan of a hurricane victim with ads for homeowners insurance.
It also means more work building those sites. Which is not something I see happening, given that in most projects there is little time to fix structural issues as is. Why fix bugs or optimize payloads when you can ship features?
I would also bet that if lower fidelity meant less (or no) ads, many people would use lower fidelity as a workaround for not having to see them. Which would be exactly what the world needs but is, again, entirely misaligned with most online companies’ economic interests.
Unless the incentives change, websites will continue to bloat until they break.