True, but how many are “enough”? Remember that Apple aggressively prune their offerings to a very small number that they judge will suit most of their potential customers well. They do not, and since after the dark days of Michael Spindler in the late ’90s, never have tried to please every potential customer, even when there is profit to be made.
For Apple, it’s all about focus, and nowadays, it seems, often about focus on products with huge volumes (with some exceptions like the iMac Pro that Apple judge to be important to their ecosystem).
There could be tens, possibly even hundreds of millions of dollars to be made selling smaller iPhones, and Apple wouldn’t try to chase that market. (I’d be delighted if they would, having just bought a brand new iPhone SE on clearance, but I don’t expect that they will.)
But a few years from now? I wouldn’t dream of predicting what the fashion in screen sizes will be a few years from now, and it wouldn’t surprise me in the least if tastes start to swing back.