Why will X not keep increasing? It’s been increasing, albeit very slowly, since the start of the 20th century as the disease burden declines and nutrition improves. I see no reason to think that future health interventions couldn’t further juice those numbers.

Will it keep up with Y? Probably not, unless people are willing to accept pretty radical interventions to their biology. But it almost certainly is not static

IQ hasn't been increasing in developed countries for about three decades now. In fact, it has been declining in most.

The increases still happen globally but mostly driven by developing countries.