Let me reiterate - the 11" Air is the 11" Pro. Same design, specs, features. Only the name is different.I have to disagree 100% with your line of thought. As soon as the 2015 12.9" Pro came out, people were clamoring for a smaller version of it and despite the incredibly small size for the 2018 12.9" Pro, people should get that choice. Plenty of users want a smaller iPad Pro or Apple would not have ever offered one.
The iPad mini is its own niche with acolytes and opponents, but should be kept in lockstep with the Air from now on as far as internals go. The current model has a small premium attached to it, but is worth it once the iPad internals are taken into consideration.
The $329 price point with routine discounting to $249 is the gateway drug for the iPad. Those who already have an older iPad that they love or who want a larger size without jumping to the 11" Pro have the Air now with premium features that fills a rather gaping hole Apple had in their lineup. To me and at least one other here @Michael Scrip the current lineup is pretty much perfect.
Consumers and a bit of advice can help make a more correct decision, but Apple has clearly delineated these products now and all of them are good...there are no duds at all. The lineup (and sales) would suffer immensely under the plan you have proposed.
But, you're actually right. It'd make more sense to just call them all iPad Pro, except for the low-cost model referred to as iPad. This is what Apple used to do with the MacBook lineup and it was very concise. On the iPhone side there has been a return to this naming scheme (iPhone and iPhone Pro) so it makes sense from a parity standpoint as well.