I really like the concept, if it can be executed at the right IQ. A dedicated TC has the advantage of being able to be optimized for the single lens, which presumably makes it easier to get better IQ. External TCs have always had a hit in IQ, with 2x much worse than 1.4x. The hit on IQ has reduced over the years with both lens and TC evolution. If this 3-setting external TC can have IQ comparable to the existing RF TCs then I think this is a winner.
Having the 100-300 and 200-500 lenses ship with no TC, but having a multi-setting external TC is a much better solution. The lens is lighter (and presumably cheaper) with no TC. The multi-setting TC will be expensive but will work with (at least) two lenses and can be added for occasions where the added flexibility matters.