There's some quotes from Phil Schiller talking about it in 2015 in this MacRumors article:
Documents highlighted by The Verge and disclosed as part of the Apple vs. Epic Games trial have revealed that Apple discussed plans for sideloading...
www.macrumors.com
We and the major game developers have tried high-end gaming on the Mac... but have failed to generate any sizable business in that genre.
Wonder what what they concluded were the reasons for it to go wrong? Too few users with powerful enough graphics hardware on the Mac perhaps? And what about a graphics API that's up to date and runs well on macOS?
I think Apple has the potential to cover both of those things now with their own GPUs and Metal API. But I also think a replaceable GPU is important for a gaming focused computer. Most often it is the GPU that will bottleneck before the CPU. If it's only Mac Pro computers starting at $5,999.00 that will have replaceable graphics in Apple's computer offerings it will be hard to to compete with what people can get on the PC/Windows side of things.
But maybe Apple isn't aiming for that and can still ”be in the game” somewhat with the ”fixed hardware” that consoles also has.