I don't see how decoherence is an automatic win for MWI. Decoherence has been used in several different interpretations of quantum mechanics, notably in consistent histories and in certain hidden variable interpretations. Why should we choose MWI before those, particularly since it seems less parsimonious than consistent histories? For that matter, the language of Rovelli and Smolin's relational quantum mechanics very nearly turns decoherence into its own interpretation (if you compare papers on decoherence which shirk the metaphysical interpretation to the interpretation put forward by Rovelli, they're almost identical). Relational quantum mechanics requires much less in the way of grand assertions than MWI and is a natural framework for decoherence, so why pick MWI over relational quantum mechanics?