The conundrum
4-wheel spinner bags: convenient, but ~10% space lost due to accommodating the extra wheels within the length limit
2-wheel bags: More space, but less convenient to roll

Solution: 4-wheel spinner bags with wheels that are easy to remove or retract, e.g., using a slide and lock mechanism (removing) or folding mechanism (retracting). This achieves the same space as in a 2-wheel, but the bag can still fit in the bin (if the bin is small; otherwise ~23-24" is usually fine) or sizer in case of an overzealous gate agent. In other words, take the same shell size used for two-wheelers and convert to four wheels, but make the wheels easy to remove/retract for the ~5% of the time you're on a small plane or encounter a try-hard gate agent.

Obviously this feature would cost more, and maybe there are some durability challenges to overcome, but with B&R, Rimowa, Tumi, etc. selling $600-1000 bags, surely they could figure something out...

Heys Stealth came closest, but it looks like they no longer sell it, and they didn't use the retractability to increase length (they kept it at 21").

Thoughts?