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Actually, I agree with you on the Physical Part. Tanking a Garchomp earthquake or something from physical Lucario is way better than surviving a special fire move or something that can't even be guaranteed.
Defense is probably more efficient you can have a dedicated special sponge support it and Mawile is still neutral to big fighting attacks/weak to EQ.
I'm thinking if you were to invest into s.def to only do it for a setup sweeper or pure support M.Mawile.