I think that when you find arguments that GC can be competitive with manual memory management, they always rely on the assumption that it is in the context of idiomatic code. For a GC language, idiomatic code allows the use of a generational collector which can automatically operate in a manner similar to an arena allocator. I suppose this is a matter of opinion, but I think that using arena allocators in a language like c++ is not strictly idiomatic - you should only add them when you find they are required for optimisation, and they have a cognitive overhead. If you make the wrong choice with your arena allocator then either you can make performance worse or you can introduce memory safety problems.

So this is the way that a GC might be competitive with manual memory management - if the benefit of arena-like allocation offsets the additional tracing performed by the GC.