Apple has only released one iPad mini the past FIVE years. They aren't releasing two within 6 months of each other. Not to mention 8.7 inch is not the 8.5 rumor everyone suggests. They aren't going to make two iPad mini sizes. They hate the iPad mini.
They don't hate it... In an interview some time ago they explained that they believed that people bought the mini just because it was the cheapest iPad. So when they decided, around a year after the mini 4 was released, to make a larger budget iPad that was going to cost as little as the very first mini they thought that nobody would buy the mini anymore, so at the same time as they launched the budget iPad in March 2017, they discontinued all mini 4 models except the 128GB one, which they priced at 399 as the original mini 4 16GB (while the budget iPad had 32GB).
Over time however they realized that people kept buying the old mini 4 despite the budget iPad, so people were interested in the size more than in the budget price.
That's why, when the resuscitated the old Air brand, to fill the gap between the 329 iPad and the 799 11 pro (that had temporarily been somewhat filled until that point by the 10.5 pro, which outlived its 12.9 counterpart) they released a new upgraded mini with the same specs. I think sales have not been bad.
Personally I wouldn't be surprised that they give us a mini pro at $500. I wouldn't see why they would keep it at 399 if they go the air 4 route. But problem is how do you name it to justify the price increase? Why not call it pro and maybe even give it A15 and quad speakers to justify the name and at that point add another $50-$100?
appletrack.org
