The iPad mini 3 is to the iPad mini 2 kinda what the iPad 3 was to the iPad 2 or the iPhone 3G was to the original iPhone. Just new features added but no improvement in processing power.
The jump from Air 2 to Pro 9.7 may not be as big as the one from Air to Air 2 but it's still significant. Honestly, we don't know when exactly Apple is going to cut iOS update support for any of their devices and for now, we can only speculate as to their reasons. Would more RAM benefit Pro 9.7? Sure. However, it's not like 2GB is super tiny. Personally, for iOS devices with 512+MB RAM, CPU performance becomes too slow for me before RAM becomes a genuine issue.
I reckon the biggest gripe people have on the Pro 9.7 is the pricing. If it had been released at $499 with $100 per capacity upgrade instead of $599 with $150 per capacity upgrade, it likely wouldn't be viewed as negatively. Is Apple being greedy? Sure, but that's the way they've always been. Their goal is to make as much profit as possible. Their products have always been on the high end of the price scale. If Apple figures they can increase price by 20% while only losing 5-10% of demand, I don't really blame them for making that business decision.