I can almost understand why people don't upgrade super old devices from some fear of their device slowing down even more, but an iPad Pro that is only 1 generation behind? Dude...
I think iOS 10 would benefit you more than you think over iOS 9. More importantly, updates are meant to help the device perform stronger, not be detrimental where it instills fear in the user from upgrading or degrade a newer piece of hardware like the 12.9 Pro.
overrated feature in my opinion, tons of people still using iOS 9 or earlier mainly because the newer versions are not compatible with their older devices. When's the last time a major iOS hack has caused havoc across the globe? If I owned a iPad 4, I'd sure as heck not throw it out and stop using it if it served my needs fine, just because iOS 11 will not run on it.