It may be hard to believe, but that's not how Apple works. Despite people thinking that Apple is a "marketing" company, they never push specs if they feel it is not needed. If specs do not make the user experience significantly better, then what is the point?
If you really want to talk about whether "higher resolution is a selling point", you can look at Samsung and various other Android manufacturers, especially Sony who have been pushing higher resolution like crazy. Did anyone really need it? Is there any real appreciable increase in visual quality moving from 1080p to 1440p? The Xperia Z5 Premium sports a 4K display in smartphone.
Seriously, if anything Apple is underutilizing the potential to produce products with higher specs so that they can produce all kinds of marketing about it.
Having said that, I don't think the iPad Air v3 needs a 4K display. Moving it to the current iPad Pro's resolution is sufficient - 2732 x 2048 maintaining a 4:3 ratio and increasing the PPI from 264 to 352.
What I'd really like to see is the iPad Pro move to "4K", which would be around 4096 x 3072 or 3840 x 2880. But knowing Apple, they won't do it until battery life and processing speed aren't compromised i.e. User Experience.