Many of us wish the display were a simple display only, no need for tuner or smart apps or speakers/amps built in....but most people seem to want a fully loaded tv and the economies of scale caters to that market primarily. I wouldn't buy anything just for ARC/eARC capabilities myself....and since I'm still just 1080p and with a 7ch speaker layout (no Atmos/DTS:X) until I update the display I'm good with where I'm at. I think mostly about better speakers as far as upgrading goes, but I'm pretty happy with my current speakers/subs and am generally happy with where I sit on the diminishing return fence for now.
As far as diminishing returns, they can be reached fairly easily I think, keeping up with the video and audio codecs can make "upgrading" as necessary as you want to keep up with such. Your avr can do quite a bit still, but if you want to run 4k video thru the avr some updating is necessary (altho a bluray player with dual hdmi outs could feed your display directly for video and you could still run audio thru your current receiver....unless you're going the Atmos/DTS:X route with additional speakers....).
Does the SVS Ultras you mention include both speakers and subs?