You get most of the economic value from the first few divisions. Going from 80% yield to 90% yield shaves 14% off your cost/unit. Going from 95% to 99% only saves you 4%.

I'm not an expert at how they combine chips. Like I said for AMD, they also wanted their units to be composable w/ small number of chips, so they basically have a die w/ a few cores and different die w/ memory, and I believe they have a proprietary communication mesh for connecting them. I think there is some considerable signal/energy overhead to communicating between chips. The cost of masks and interconnects is probably high enough to make a high number of diverse chiplets inviable, but I wouldn't say it's impossible in the future.