IMO manual memory management is the epitome of sunk cost fallacy. Sixty years of good programmer effort thrown at a bad idea that has produced an endless stream of security bugs and performance problems that have only hidden the cost of memory management behind non-obvious barriers and done severe violence to systems languages with an absurd obsession with custom allocators and baked ownership into otherwise straightforward APIs and made all programming harder.
> we have a giant for loop that iterates over all of memory over and over and over
I don't think you understand how garbage collectors work.
How is that not how garbage collectors work? You start at a gc root and chase pointers all through memory. Then do the same thing again and again. (I'm assuming we're talking about a standard, generational mark-and-sweep gc.)
>> we have a giant for loop that iterates over all of memory over and over and over
> How is that not how garbage collectors work?... I'm assuming we're talking about a standard, generational mark-and-sweep gc
GCs do not scan "all memory", but small fraction of memory. In case of generational GC the scanned fraction of memory is (usually) limited to single generation. Even without generational approach scanning heap itself is frequently avoided in favor of scanning separate data structure with highly compressed representation of object set.
GCs do not generally iterate over memory just because they can. They either reclaim space for new allocations, move things around to reduce fragmentation or fire periodically in response to increased allocation rate. If your program does not make allocations, it may never incur a GC at all.
The grandparent comment makes it sound like garbage collection is a simple effort, conducted solely by distinct GC code ("giant for loop"). This is often not the case: for example, JVM may generate additional memory barriers in any code, that uses references (exact nature and purpose of memory barriers depends on GC being used [1]). Augmenting the code with those barriers allows GC to operate more efficiently and quickly: achieve smaller pauses, scan less memory, collect memory for some threads without disturbing others.
While I can't agree with the GP at all, I also can not agree with you. There are more options for memory management than fully manual and fully runtime.
> we have a giant for loop that iterates over all of memory over and over and over
I don't think you understand how garbage collectors work.