> Not thinking about ownership over references to heap allocations is very very much the big productivity win from GC.

I very much doubt that this is a big _productivity_ win. Languages in which it is idiomatic to "think about ownership over heap allocations" (C++, Rust) aren't obviously less productive than comparable languages where such thinking is not so idiomatic (C, Java, .NET, ObjC, Swift etc.).

It's somewhat common to use refcounting (shared_ptr<>, etc.) in the more exploratory style of programming where such "thinking" is entirely incidental, but refactoring the code to introduce proper tracking of ownership is quite straightforward, and not a serious drain on productivity.