MCM is a old trick in the toolkit, eg Intel x86 and POWER chips have used it on and off over the years, as have various game consoles. The downside is the cost and complexity of packaging, and the power required for inter-module communication. Many of the past usages have been to closely couple a die from a DRAM process and another die from a logic process. It's not obvious that MCMs are becoming more popular on average in the future.