Also "effectively limitless" means equivalent to the specs of the worst system the designers use, which will likely skew things. Where I'm sitting we share 384K down / 128K up between about 40 people. Updaters that refuse to stop downloading when you close them really bog down the network. Now, you could try and put in some custom menu to control that sort of thing, but that's going to be a mess of configuration and hard choices, and developers will have to do that for every single app.
Quit/Close are two well-defined commands that give the user a well defined way to say when they want something to go away vs. when they want something to go away and quit doing anything. Re-implementing "go away and quit working" on a case-by-case basis is going to end up with every app either having a nonstandard interface for doing so, or no way to do so at all (like most update managers.)
Personally, I think "some apps don't quit properly" is a description of a problem with the apps, not with the "quit" command. Comparing to mobile is a red herring, because you can in fact quit background services on most mobile platforms, and it's a necessary thing to do. It's only programs that by design, always gracefully suspend, full stop, and resume later that make sense to not have a quit command.