I've been playing PC games since 1998 and I've played them exclusively until 2018.
They heyday of PC games from a market share perspective was in the past (probably around 2005?). At the time there were just PCs and console, I think they were reasonably close.
Now desktop games are less than a third and they will probably be around one fifth or less within a decade.
That doesn't mean that they will be bad, the overall PC game market value will grow, it's just that the mobile game market will grow much faster and become much, much bigger over time.
Mobile gaming is surely huge and growing. But I don't see how it has much bearing on the health of PC gaming. It's not a zero-sum game. I can buy that PC and Console gaming are in competition but the gaming experience between mobile and PC/Console is vastly different.
Sure mobile gaming is massive. But considering the current growth of PC gaming it's hardly something that negates the existence of PC gaming even if it's smaller in comparison.