Lots of GC vs Manual MM do a disservice to manual MM, by comparing GC to a strawman with lots of malloc/free.
IME, manual MM often embeds allocations within each other and/or places allocation on the stack. Both of these options make manual MM far cheaper than GC, rather than slightly cheaper as shown in these papers.
There is nothing that prevents a GCed language to allocate things on the stack. Also I'm afraid using std::string in C++ causes more allocs/frees than typical use of Strings in Java/C#, because the former can't be safely shared and must be copied - on the heap.
A GCed language requires good stack escape analysis. Unless you expose it to the programmer for at least semi-manual MM, it is bound to have false negatives, where you pay for the allocation on the heap unnecessarily.
I totally agree about std::string.
I think C++ with the conventional libraries is actually an example of how to do manual MM badly.
IME, manual MM often embeds allocations within each other and/or places allocation on the stack. Both of these options make manual MM far cheaper than GC, rather than slightly cheaper as shown in these papers.