The real question is why the developers (or just Gavin) are focusing on changing the block size limit or block generation time instead of this?
I am actually somewhat surprised Gavin has made that suggestion as increasing the probabilities of an orphaned block will encourage miners to propagate empty blocks for a slight block reward edge undermining his efforts to increase tps. Another concern is it will encourage centralization in pools to reduce orphans.
Perhaps he was just giving a nod without thinking about the implications in depth as he really isn't too interested in mining as he has admitted multiple times..