This is simply not true. This sentiment is being perpetuated by an antiquated, old-fashioned view of the Mac.
In an interview with Apple’s vice president of Platform Architecture and Hardware Technologies Tim Millet:
“My team spends a lot of time thinking about how to make sure that we’re staying on that API curve to make sure that we’re giving Metal what it needs to be a modern gaming API. We know this will take some time. But we’re not at all confused about the opportunity; we see it. And we’re going to make sure we show up.”
Consider eGPU support in High-Sierra, and also the aforementioned development of Metal. Finally with Apple Silicon, we have a real potential for some serious CPU/ GPU horsepower. They're already at 32-38 GPU cores. I remember reading an article on the plans for AS raytracing as well.
The future for Mac gaming has definite potential. The key is breaking the aforementioned stereotype and breaking the cycle where developers don't want to make games for the Mac because there are no customers, and customers don't want to game on the Mac because there are no games.
Firstly Apple needs to make the hardware that can game. We're just seeing that starting now. Secondly, Apple needs to convince the developers. Apple's best strategy is to purchase a major game studio and get them to port their games to Apple Silicon to get the ball rolling and show the potential.
"Build it, and they will come."
I personally would love to ditch my gaming PC and just have one computer that does everything. World of Warcraft looks amazing on my MacBook Pro with the HDR support and ProRez display.