First "myth" is wrong. Loading CSS and JS with every click isn't about being "slow" it's about being efficient. CSS and JS requests add a non-trivial load to the server. So if you have 10,000 clients making requests for content they already have cached (they're just checking to see if these assets changed) then you're wasting server resources when you could've just kept the single page active and requested just what you needed over an AJAX call or even better: A WebSocket.
It's essentially why people have moved away from server side rendering.
It is way less resource intensive to send just the data and let the client do the rendering. Both in data transfer and compute.
im not aware of any content oriented websites that are switching to client rendering.
look at any news website - all rendered on backend, because it makes the most sense for composition, SEO and performance.
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