DAC improvements slowing down?

Low jitter is worth it. Many ways to achieve that these days for reasonable cost if done right. Has probably only gotten to that point in recent years, so older gear in general will tend to be more problematic.

"Beyond that how are DACs going to be a game changer?"

The same way phono cartridges have been for many years.

Until someday when digital is replaced. That could be a very long way off or never. I have no clue what would replace it. Holography like on Star Trek would be my guess (still digital but not just audio) for the ULTIMATE music simulation. I hope I live to see it. It will pretty much just continue to get better and better. Needs of teh fussiest audiophiles in terms of sound quality will become trivial problems to solve on the grand scale. BEcoming more that way already. Of course the most driven audiofiles will never be satisfied....

Jitter is to digital sound much like focus is to digital photos.

No digital photo has perfect focus, but the percent of error can easily be so small with modern digital cameras that practically it does not matter.

Same with jitter in time dimension with digital sound. Below certain thresholds, practically the smearing of the sound that results will not be detected or matter. It is a form of distortion though, so it is never a good thing.