In hindsight I think having Jason say this in Season 1 was the showrunners way of telling us Johanna Lannister was never going to show up
When I first saw the scene, I thought it was kind of a joke about his future wife, like it would be something we’d look back on and laugh at once Johanna showed up. But after Season 2, I think it was actually a hint that she was never going to appear.
They could’ve had any other character say that line, including his brother, but they chose Jason. That feels like a deliberate choice.
And even if you say, “oh, Johanna only becomes important later, of course she wouldn’t show up now…”, and yeah, she does become more prominent during King Aegon III’s reign. But her absence contributes to two of the show’s biggest issues: not giving enough importance to other houses and not showing why we should care whether certain characters live or die.
In GoT, the stakes feel much higher, and character deaths hit harder because we were show for what and for whom they’re fighting for, and who will be mourn their loss. If you watch the Red Wedding in a vacuum, you might feel sad, but after getting to know Robb and the Starks, and understanding how much the others will suffer because of it, it becomes absolutely devastating. In contrast, HotD doesn’t really give us that, why should anyone care when Jason Lannister dies, or even if House Lannister and the Westerlands were completely wiped out?
It wouldn’t fix everything, but having Jason interact with Johanna and their children could’ve helped develop his character and establish hers, especially if (big IF) the show ever explores the regency period. That way, she wouldn’t just suddenly show up and leave people wondering who she even is