While they could improve PPI, the display is the last thing I would change in the iPad Pro.
Colors are Good, resolution is more than enough unless you are really looking for problems as the viewing distance is enough to mitigate the “lower” resolution.
Personally I never had issue with the res, but that’s just me.
The iPhone and iPad are used at a similar distance, so it make sense that the iPad has a similar PPI too. And I do notice the lower PPI sometimes. The iPhone is always 100% sharp all the time.
However, I understand why Apple wouldn’t do this as it would raise the costs of the iPads to an absurd amount. And the price of the current iPads is already very high.
The iPhone is simply Apple their premiere flagship device which has no compromise (which is why it cost similar to an iPad while being much smaller).