To be fair Francis Stephan had qualities of his own, for instance he had a much better grasp of finances than Maria Theresia. Not to mention, that his, but also Leopold Clément's upbringing was closer to the traditional upbringing of a future ruler. For all the effort Charles VI put in the Pragmatic Sanction, he apparently didn't feel the need to aim to steer the upbringing of his daughters a bit more towards, that of a future ruler (they had the 'normal' education of an archduchess); OTOH Maria Theresia turned out pretty good.
However you're selling Francis Stephen short, during their shared reign he was responsible for the financial affairs and was very competent at it, he also assisted Maria Theresia in other matters of state, but was content that ultimately his wife was the one wielding power.
A potential more active Leopold Clément might not necessarily have made for a better outcome; IOTL Maria Theresia and Francis Stephen were quite compatible.
I agree that Leopold Clément like Francis Stephen would probably also have to renounce Lorraine for suitable compensation.
@ Emperor Constantine: instead of obsessing over that 'worthless piece of paper', Charles VI IMHO would have been better off, if he followed the advice of Austrian field marshal Prince Eugene of Savoy to fill his coffers and build up his army.