Why a larger than 30" iMac? Why not a Mac Studio with one or two high-end monitors?
I can think of one good reason. I hope it is true. An iMac does not need an HDMI or Thunderbolt cable and should not be limited by the HDMI or TB interface. With a direct, no cable interface to the GPU the speed potential is not limited.
I can imagine an 8K screen running at 240 frames per second. Maybe they will connect the GPU directly to the LCD panel and build something with video we've not seen before.
Display Port 2.1 can already 240 Hz at 8K (if at slightly decreased colour fidelity), but 120 Hz is fine. I don't think the connection is the limited option. At 5K, 240Hz is easily with DP 2.1.