The air 2 doesn't count since back than there were no pro or budget iPads and it was the top of the line...Also fair points. Like I said, it all depends on when the next Air releases. It’s expected next year. I guess it’s likely to be released in the first half of 2022, so if that’s the case, it very well could receive the A15, or some sort of variant like an A15X, similar to how the Air 2 received an A8X and not just an A8. Who knows…
At this point it seems clear:
Base iPad... updated yearly, it gets old A chip (non X) and keeps it for 2 years
iPad Air... updated every 2 years, it gets current A chip (non X)
iPad pro... updated generally every 1.5 years, it gets AX chip (now M)