After 30 years, do the math of much years are left.
I'd do the math, but it really depends on whether the recent growth holds, accelerates, or slows down again. So, hard to tell.
The moment another or a few new AAA titles with kernel-level anti-cheat come out, people go back to Windows.
Gaming could, and probably will, be the key to the "Year of the Linux desktop" but the anti-cheat problem needs solved.