We are still trying to find a stream of the full panel, but a few people that were at the event have confirmed this one. Apparently over at PonyCon Holland, Gillian Berrow revealed a very interesting little fact about why we might not see a whole lot of crossover between the universe of G4 and G5. Apparently it all comes down to licensing and usage. Discovery Family still owns part of Friendship is Magic's show content, which lets them charge fees whenever a G4 pony appears in something like Make Your Mark. Twilight as a specific example needs to be paid for whenever she makes an appearance.
We will try to dig up the specifics on this one, but for now it does explain why they seem to light on the throwbacks. It's always a shame when corporate nonsense butts in on creative things.