You’re right to say that Apple have previously gone by chipset generation when dropping iOS support - the A5s and A6s have so far all been dropped together, but I really can’t see them cutting off this iPad that early, especially as it’s catering to the basic iPad crowd that might just now be updating from iPad 2s - more likely would be giving the 6s series an extra year of support (they are also still being sold, as is the SE) - so we might see another year or two where no devices are dropped between when the A8s go out and when the A9s go out.
I guess you’re right in saying there’s no guarantees, but I’d like to think Apple would continue offering the generous support they have been known for until now.
A9 will be supported though iOS 13, which is 4 years after launch of the processor. For a low cost iPad, I think that’s decent.
I feel bad for the iPad Pro 9.7” owners however. The A8X is a 2014 product so suppprt is expected to last through iOS 12 only.