It’s not just about Retina displays. High res and HDR isn’t uncommon anymore. Pretty much all new TVs anybody would want to buy will be 4K. It has to support the Apple 5K display anyway because that’s one of their products.
As we’ve discussed two external displays are supported on the new macbook base models. It was a bit of an oversight on the original sure, but that’s been fixed now.
Also the same SoCs is used in iPads. It’s not mac only. I can’t imagine wanting three displays on an ipad.