FWIW, I haven’t found this to be a problem in practice, and I don’t know of any package where being able to precompile these blocks would give a significant speedup; @requires blocks are generally only a few lines of header-like code.
Being able to auto-import packages is a nice feature of the static approach which would be useful for optional backends. It wouldn’t be able to handle all cases that the dynamic approach can; for example, if you load a package then install and load a conditional dependency of that package without restarting. I think the only full solution is conditional sub-modules that are separately precompiled and handled specially by the package system.
I would personally favour the support for require hooks in base as a simple short-term solution, and then figure out something better. (I’m not sure why you think this would cause an issue with cache files though; the require blocks aren’t cached so there’s nothing to change if the set of available deps changes.)