> plug in a garbage collector.

Garbage collectors don't guarantee the absence of memory leaks. GCs remove one important source of memory leaks but it's still very possible in GC languages to use up all available memory unintentionally simply by holding onto things in a big data structure that you've forgotten about (often it's a cache). Weak pointers in conjunction with GC help a great deal with that problem but even so GC and weakness are not going to guarantee leak-prevention in all cases.

I still strongly prefer GC languages to the alternative.