Agreed!
Apple has more than enough resources to support older models of their installed phone base, and it would make good sense to keep these legacy users happy for the eventual upgrades and "not to switch platforms". Most legacy phone users are probably using many other apple products as well and the cost of pushing upgrades on iPhone's will impact future purchases in other areas like ATV, iTunes, iPads, and especially MAC's
Why not sign and support older iOS versions? and not use "security" as an excuse to push everyone forward to a new iOS and thus a new phone
R&D for cars, new emolji, AI, homekit, are fine but how about showing some loyalty and appreciation for, past buyers and supporters of Apple?