Virtual memory can be used as "poor man's GC" only with languages with a variable number of bits in pointers (references). If you have a fixed number of bits in your pointers, like in C & C++, then you will run out of addresses eventually, even with virtual memory.

Yup! Virtual memory as a poor man's GC seems to be viable for short lived programs with modest virtual memory requirements. I seem to recall Walter Bright using something along the same lines to replace malloc/free in the D compiler code. It lead to a significant improvement in performance.

Copying garbage collectors benefit from virtual memory.

Copying GC needs two same sized memory pools. One is used and one is empty. If you have same amount of virtual memory as ram, the empty unused one can reside in the virtual memory. When GC happens live data is copied (and compacted) from live pool to empty pool and the roles switched. With virtual memory, you can use as much ram as there is without affecting performance significantly.

If you do manual memory management, long running programs cause memory fragmentation. If allocated memory sizes are very variable in size, available memory can be half of the actual memory in the long run.

In theory, moving and compacting GC with virtual memory can almost double the available ram compared to manual memory management.