It’ll be similar to last year where Apple will hike the price up, less people will upgrade but the revenue will be even or higher due to a higher retail price covering the gap. I think a lot of people, myself included are seeing less reason to upgrade and are even buying last years models to get the Pro features but at a more reasonable cost.
OK, there may be concern about a probable price increase. However, what is being suggested in comments here (is as you suggest) that the increase in quality/facilities is not regarded as worthwhile; diminishing marginal utility has set in; the tech seems to have reached a plateau, or at least no wow factor is anticipated.
iPhones really are splendid products, highly versatile and durable. Mine does its job, just like my fridge and washing machine. I don't replace those that often, albeit that they don't accompany me outdoors as a fashion statement
All this leads me to think that Apple won't secure much, if any, extra revenue (PxQ) from upgrades, at least from the large segments of their existing user base. That said, many new markets might be opening and, of course, there are new labour market entrants every year. I splashed my first wage on hifi and music - the smartphone is now the route to that and more.