I think people don't realise most 4K TVs already upscale 1089 > 4K, so why would you need the console to do it?
It's patently untrue that there's no benefit to the console doing native upscaling. There are multiple scenarios where it really would make a difference.
For one, games with a frame buffer at any resolution other than 1080p. While there almost certainly isn't the horsepower to run AAA titles at 4K, there may well be a few that decide to run at intermediate resolutions. 2400 x 1350 is about 56% more pixels than 1080p. Scaling down to 1080p and then back up to 4K wouldn't offer any resolution benefits, but scaling directly to 4K would be a different story.
For another, even at 1080p the UHD specification provides significant per-pixel benefits over HDTV. Support for HDR and much improved color gamut are both perfectly good reasons to support 4K output directly. Converting the same signal through a standard 1080p intermediary would strip all of these benefits.
Lastly, even if not very many games support native 4K, that's a very different thing from
no games with a full 4K frame buffer. Having output hardware that allows a seamless transition between AAA 1080p titles and less intensive 4K titles without having to have your AVR or TV resync across resolutions changes makes for a much nicer experience.