I think that the iPad Pro 9.7" will be replaced by a new iPad Pro 10.5" and the iPad Pro 12.9" will be replaced by a new iPad Pro 12.9". Then the iPad Air 2 will remain as the low cost option. The current iPad Pro 9.7" is not a low cost device. I think that the Air 2 is more like the iPad 2 in that it will stick around for a few more years as the low cost, but very capable, iPad.
This gives 9.7" in the Air 2, 10.5" in the smaller Pro and 12.9" in the larger Pro. The latest models would then be 10.5" and 12.9", but the low cost option is still available at 9.7".
7.9" is a handy size for reading, handbags, travel and kids. I have both the larger Pro iPads and passed the mini 2 to my daughter. I do really like the size though and would love to get a 256GB one with true tone, better cameras and pencil support. It is overkill though and probably feel guilty for having 3 iPads. It would make an awesome eBook reader and travel companion though. I think I'm trying to justify buying it already.