Explain the Many-Worlds Interpretation (MWI) of quantum mechanics, focusing on the resolution of the measurement problem without wavefunction collapse. Discuss the concept of 'decoherence' and how it explains the appearance of definite outcomes from the superposition of states. Address the ontological implications of MWI: if every quantum possibility is realized in a branching universe, what does this imply for the concept of probability and the 'I' that experiences a specific reality? Critique the argument that MWI is ontologically extravagant compared to collapse theories.