The thing is you can still stick a CDN in front of your old school servers and just use a 'stale-while-revalidate' header to get exactly the effect described here.
I already had HAProxy setup. So I have added stale while revalidate compatible header from HAProxy. Cloudflare handle the rest.
Edit: I am not using vercel. Self hosted using docker on EC2.
Now that I have proper header added by HAProxy, Cloudflare cache rules for stale-while-revalidate works.
If anyone can reach Cloudflare. Please let us forcefully use stale-while-revalidate even when upstream server tells otherwise.
I obviously can be done but clearly is not the intended solution which really bothers me