You didn't even know what iOS 11 was going to give you in the first place when you bought your "1,400$" iPad. Sorry but you're never going to be happy in the technology world if you think you are entitled to all software features that were never promised nor announced.
You got exactly what you bought, it has nothing to do with Apple or planned obsolescence. This is the nature of every technology products with no maintenance policy, you're not entitled to any future features unless it has been explicitly promised by the company. Apple should've explain why it was in the first beta and pulled but they're not obligated and they suck for doing this for sure.
Macs have the same problem, I remember certain features were not made available to certain macs like '12 rMBP in future macOS updates even though there were user patches that made the said features available and working fine. I forgot what it was, AirDrop maybe? I don't recall but Macs/macOS definitely isn't immune to obsolescence.
The problem is, Apple try to sell their iPad pro being as good as buying a full laptop, so yeah, the one year old iPad pro 12,9 should be supported with full featured software updates just like new iMac are for years. The first iPad pro is still a perfectly powerful device so there is just no excuse.
But that being said, I still don't understand people buying tablets to use like a computer even if Apple wants you to believe it is. I never bought an iPad for that purpose and my iMac was and always will be my real machine for more than light web browning and media viewing. And iMac even the lowest one, is just a little more $$ than an iPad pro with keyboard.