If you think yours is performing well enough, keep it. I have a Mini 2 and it's painfully slow when doing certain things. ForeFlight takes a pretty long time to launch (just measured - 26 seconds!
) and if I don't pause after every fix I enter in the flight plan and wait for it to look up the fix in the database and add it, the keyboard will drop the next 1-3 characters and mess up the next fix.
I think the biggest argument for upgrading right now is that it's been four years since the iPad Mini 4 came out. I haven't bought a Mini 4 because by the time my Mini 2 got slow, I didn't feel it was worth the investment to buy already-outdated tech. If they wait 4 more years before the next upgrade, I want to have gotten plenty of use out of it in the meantime!