While I agree higher PPI would be nice, if Apple follows their existing process, we'd be looking at what it takes to make the iPad 9.7" a 3x device (instead of the 2x it currently is) so that on screen elements remain the same size as they do currently. To do that, we are looking at a 396 PPI device, and the resulting hardware to power it. That's about 2.2x the current number of pixels. So I'd want a device with the following changes to maintain current functionality:
1) 4GB RAM minimum (or get rid of split view).
2) A GPU with double the performance (or take a perf hit).
3) A CPU with double the performance (or take a perf hit).
At some point you will reach some sort of breaking point where it will make sense, but I'm not really sure that is today. And even if this all happens in Sept, a question certainly could be: do I want to spend this extra headroom on a higher PPI screen, or something else? This last year, the headroom was spent on split view. I'm actually glad for that. Also, Apple tends to aim for as much being pegged at 60fps as possible. I don't really fault them for that, either. Especially since they tend to get flak when it doesn't. (And I'm thinking of the 10.0 / 10.1 days which were really bad)