What is even the point of garbage collection? By the time you manage all the resources of a 100k line application you're going to have to do as much work as if you wrote it in C.

Well the point of gc is to allow the algorithm to focus on the problem domain. and not worry about mechanical implementation details of a computer. Everything has a trade off. An obvious win still must pay a cost elsewhere. In the case of GC it's simply not a good trade off for many resources. Sometimes it cannot be used even if you were willing to accept a trade.

Managing resources by hand is managing the mechanical implementation details of a computer. So in order to have garbage collection that manages more than just a sliver of memory so small that it counts as a rounding error by the same you get to the size of real world projects, you need a custom operating system and possibly even custom hardware too.

Your statement is false for many real world projects. (For some, you are correct. Those projects should not be written in a garbage-collected language, or should be written to run on a custom OS and custom hardware.)

When GC'd objects represent resources like open connections and files, use-after-free safely and reliably raises an exception to handle. Without GC, use-after-free on pretty much any object is so catastrophic that even correct code starts doing unpredictable things.