Writing backends that can actually handle public traffic and using authentication for expensive resources are fantastic alternatives.
Also, cheaply rate limiting malicious web clients should be something that is trivial to accomplish with competent web tooling (i.e., on your own servers). If this seems out of scope or infeasible, you might be using the wrong tools for the job.
You still have the network traffic issues which is very substantial
Even if you write the best backend in the world where do you host them? AFAIK Cloudflare is the only free CDN.
GitHub pages?
You can't pipe any media files from any origin to GitHub Pages.
There are many free static site hosts but not many free CDN.
If it were this easy, we wouldn't have had about 10 HN posts on the topic in the last few months.
The technical skills of the majority of the HN community are way below those of the typical computing community a generation ago.
This sounds pretty unrealistic: the web is not better off if the only people who can host content are locking it behind authentication and/or have significant infrastructure budgets and the ability to create heavily tuned static stacks.