One could argue that engines with gearboxes will never succeed
Oh no, you couldn't. Or you'd have to convince me the Merlin, Jumo205-208, Wright Cyclone &c &dc did not succeed. And yes, all of these are big engines, yes, but there was the Pobjoy, too. Gearboxes have been around for decades, nothing new or difficult about them. They do work better at high rpm's and with lots of cylinders, though.