My goal was to emphasize that you are correct that not all useful instances of dynamic dispatch can be reduced to static dispatch. With regard to the DOM specifically, however, there is some concern that the structure itself was designed and standardized such that it can only be implemented efficiently by assuming some sort of inheritance scheme, effectively baking the typical OO approach into the web standard. Opponents of inheritance being added to Rust have claimed that this is a unique case, and that such a feature would not pull its weight in any non-DOM scenarios (or worse, that it would be redundant with Rust's trait system and that you would fall into C++'s trap of nobody agreeing on which subsets of the language to use). Given the nebulousness of the proposals in this space, I have no dog in this fight at the moment.

My other goal was to show that Rust gives you enough tools to manually emulate an efficient inheritance-style dynamic dispatch scheme, even if it can't fully prove that your implementation is safe.