> at a certain point the consumer doesn’t want to buy an apple the size of a grapefruit Um... yes I do. Is the Honeycrisp itself not sufficient proof that apple growers have had absolutely no clue about what consumers really wanted, for decades? In the same vein, where have all the honkin' big seeded watermelons gone? You remember those, back when they had big black seeds, and actual flavor ? Don't blame the consumer…
Do you have any idea what edible mutualists you could grow with them? I'd bet there is a fungus meant for the rootbed. Having trouble intuiting much beyond that though. I agree with what you're trying to communicate, I think, but my anecdote from yesterday disagrees with size. I looked at the massive honey crisp on my counter (around a softball's size), but didn't want to have to cut it in half.
What I don't want is one apple that's one size fits all. Because the last variety they tried for that was Red Delicious, and it is the worst named variety of apple I have ever eaten. The only way to get worse is to try naturally pollinated grown-from-seed apples, and even then you have to be very unlucky with your pick.
Apple farmers tend to graft the variety clones onto dwarfing rootstock, to control for height and branching, so it's likely that any mutual crop would work just as well for any kind of apple. I seem to recall that someone was trying to get truffle to grow in apple orchards, but I don't remember seeing anything about it actually being successful.