One thing no one has commented on, was heat generation?
Less valve area and lower rpm potential.
Engineers may have realized that people don't spend much time on the top-end in a non-racing vehicle.
You're right, RPM potential generally decreases as you increase reciprocating and rotating masses.
I don't think they changed the red-line did they? They may have offset the larger pistons with design/material improvements, as well as dropping .1 liter. Much lighter crank is also something to consider.
But your point may be, if they applied similar improvements to the quad, additional performance improvements may be observed.
BUT, if you can get the same performance with lower part counts, why not?
The chief purpose until recently for even cylinders all these decades has been for balancing and smoothness. One trade off to consider with decreasing those masses, is a potential decrease in torque; valuable for "getting off the line", low-end performance and being able to cruise up highway inclines without relying "too" heavily on turbo. Though 3-cyl engines were going in mid-90s Geo Metros and some later model performance motorcycles, I'm going to guess that dual-mass flywheels are playing an important role in the decision to drop a cylinder. Unfortunately DMF may be adding another significant failure point when it comes to performance vehicles.