The share of all current models is 61% (iPhone 8/8+ and X) - the rest is older models. I think that‘s a good indication that Apple has reached its price cap.
20% consist of a model $200 more expensive than any previous model ($300 more than the standard-sized iPhone 8). Maybe $999 is the cap you're referring to? Getting 20% of the user base to trade up seems pretty impressive to me.
As to the sales of lower-priced models? That's always the case. The glitzy new models draw people to the stores, then their budgets force them to compromise. "It's still a great phone!"
One way to spin things is that, while similar numbers of budget-bound buyers couldn't justify paying more, a substantial number of more-affluent buyers had not yet reached their cap.