From the initial material that Canon released, it seems that a major advantage of the RF mount is the ability to design the lenses with far greater rear elements. The assumption that Tele RF glass can't be made smaller just because of the flange distance may or may not be true. But there's definitely more to it that just the distance.Maybe... But maybe the shorter flange distance (same mount diameter) is a gain that balances out any compromise. Guess we will see.