Not sure why you're getting downvoted, you're dead on.
I just finished routing an LPDDR4 bus (high speed + low voltage => worst case signal integrity) between two BGAs with signals routed on only 3 layers. The catch is that you need each high speed bus to be comfortably sandwiched between solid copper planes. Layers dedicated to reference planes start to add up quite quickly when you've got RAM, eMMC, ethernet, wifi, camera input, display output, SD card, etc all crossing over and through each other.
I just finished routing an LPDDR4 bus (high speed + low voltage => worst case signal integrity) between two BGAs with signals routed on only 3 layers. The catch is that you need each high speed bus to be comfortably sandwiched between solid copper planes. Layers dedicated to reference planes start to add up quite quickly when you've got RAM, eMMC, ethernet, wifi, camera input, display output, SD card, etc all crossing over and through each other.