The 4098x3072 that you're asking for is 125% more than the current iPad Pro 12.9" and a 249% more than the iPhone Pro Max (due to how much smaller the iPhone Pro Max screen is vs the large iPad Pro) which is not as insignificant of an increase in pixels as you suggest.So the iPhone can handle an extremely high ppi but the iPad's battery is too small? Riiight....
Just pointing out the math mind you, I'm not against an increase in resolution as long as it doesn't come with a significant drop in battery life, particularly given how variable standby battery life can be for users depending on how their iPads are set up.