Is it realistic to call the entire site asynchronously without generating static HTML?

Question

If a website, all data on the front-end and back-end are completed using ajax asynchronous interaction, is it realistic?

I have never done a large project, I am just a beginner, and I don’t know how much the server overhead is for static html and full-site dynamic interaction.

The whole site interacts asynchronously. What should I do if I receive an attack?