The exact tracing work needs to be able to know a little bit about AtomWB, just like it knows a little bit about Atom. Since the AtomWB methods are closely tied to eg AvmCore::atomWriteBarrier{_ctor,_dtor,} it's a major pain to move AtomWB outright into MMgc. Instead, we can create a base class in MMgc that has the necessary functionality and from can be extended by AtomWB.

Why not just move atomWriteBarrier* from AvmCore into MMgc? (And provide inline wrappers in AvmCore)

That was the other possibility that was discussed. The subclassing seemed like much the smaller change and was attractive for that reason.