The developer can only know this to the extent that they control everything happening in memory. The moment libraries maintained by disparate teams enter the pi

The developer can only know this to the extent that they control everything happening in memory. The moment libraries maintained by disparate teams enter the picture especially if they are binary-only or otherwise encapsulated, defensive coding starts because you can't reason about lifetimes across the whole program anymore. It also massively complicates any API surface as now every API must declare its refcounting and ref holding rules. GC gets rid of all of that complexity because the GC actually does see the whole program.