The larger die to spread out the active parts of the chip may help. I doubt that the high core chips will clock to neat 5Ghz anyway. Don’t forget that the TIM on both the Skylake X and the Kaby Lake chips is sufficient to run the chips at Stock speeds. Overclocking is not something that is a guaranteed feature. Also, solder would significantly increase the price of manufacturing the chips. How much noise was there about the $1700 6950X?
Keep in mind that solder is not the only approach that can be taken to effectively transfer heat from the die to the heat spreader. Delidding is replacing stock TIM with a different type of TIM to get an improvement after all.
Intel also has the need to use a material that will remain stable over the life of the chip. Liquid metal, by its viscous nature, is not likely to remain stable and in place on the die for the next 10 years without the need for reapplication. It is also conductive so you are always at risk of shorting components under the spreader if it does leak out. not something you want if you have to warranty a product.
What seems to be the core of the issue is the silicon glue that is being used to attach the Heat spreader is holding the spreader too far away from the die to be really effective when clock speeds are increased over stock levels. I suspect that with closer heat spreader to die tolerances, the standard TIM may even be sufficient to run the chip out of spec. Intel needs to look at ways to improve the way they attach the heat spreader so that it sits with tighter tolerances to the die.
Maybe a better adhesive may be a thin layer of CA Superglue instead of the silicon?