The MB.3 was Sabre-powered, which meant low-altitude, expensive and unreliable (it killed Baker) - and by the time Martin mated an evolved airframe to a Griffon and contraprop to make the MB.5, everyone and his aunt knew that it was late in the day to start a new piston-engine fighter.
And going back to the 1938 MB-2 with the Dagger, although not an M.20 competitor, I was reluctant to pitch it in to the "what if'" runaround.
Having said that, Jimmy Martin certainly put great stress on easy maintenance accessibility for all of his fighters.