All I want is user-isolated (suexec) shared web hosting on scalable web servers and SQL servers that someone else runs for me, where the extent of what I need to do is put PHP/Python/shell/whatever files in a directory on some network filesystem, and optionally wire up FastCGI if I want. It works well, and it's been doable for years because I used to run infrastructure like this. In the modern era you can even imagine extending suexec to create containers and not just switch user IDs if you want more isolation - as long as you have the container image locally, the individual steps of making a container (make some namespaces, make some cgroups, set a seccomp policy) shouldn't be noticeably more overhead than exec / process startup itself, and, again, FastCGI is an option.
That's "serverless" just as much as anything else is (i.e., the person running the website doesn't need to think about launching and operating servers, the platform takes care of it) and it's way simpler than using all these fancy tools on top.