It seems that it can't be a coincidence that they decided to stop reporting units sold last quarter. They must have seen this coming. And maybe wanted it to be this way (sell more fewer but more expensive devices).
I think they could easily increase the number sold if the released a new iPhone SE. But I wonder if they've calculated that an iPhone SE would actually decrease overall revenue, cannibalizing sales of their expensive iPhone XS and XR phones.
With the iPad they've aggressively gone after all parts of the market (from the entry level 329 model low-end which often sells at 249 to the iPad Pro 12.9 starting at 999).
I have to assume it makes financial sense for them not to target a lower end market with the iPhone SE? That they would see no reason for these new, big expensive phones.
But I think consumers would really like the option.
I am certain that Apple have deliberately discontinued the iPhone SE for the reasons you say: it would cannabilise the much more expensive iPhones and lead to a drop in revenue. But that is the ethos of Tim Cook's Apple: Make Money, Don't Worry about Quality. I preferred Steve Jobs's approach: Make the Best Possible and the Money Will Follow.
Unless Apple bring out a follow-up to the size of the SE, my next phone will be Android.