The benefit should be completely obvious. Having a single integrated system is always going to be more reliable than running 2 separate machines that need to cooperate. Otherwise every issue needs to be isolated to the TV, the streaming device, or the connection between the two.

Also hand waving about "new codecs" doesn't hold any weight. A Roku/Fire/Google TV is a Linux computer with hardware acceleration for all modern codecs. By the time AV1/VP9/H265 are being replaced in the 2030s, you're going to need a new TV anyway to support 8K and whatever other features are de rigueur then. Same goes for audio codecs and Wifi support.