Hmmmm.... lighter, smaller. You didn't get the memo?Of course; we've all seen that. New RF lens: lighter, smaller, ?? better optics. If Canon wants to signal that the R series is for demanding pros, they need the glass that serious pros will want to use. See the new 50 f/1.2 as an example.
Pros are smart -- and cheap! They don't buy new glass for the sake of new glass but because it fills a need. Sure, someone with a new EF 400 f/2.8 III isn't buying the RF anytime soon. But there are lots of working pros who would be in the market for new glass. Canon needs to have something for them.