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I don’t know how to really answer this without going geek mode.....
Any rotating solid shaft has what’s called a “critical speed” where the shaft will deflect at a given rpm and cause the shaft to bend away from the axial centerline in a harmful way to power transfer. The load, diameter, length, balance and stiffness of the material the shaft is made from, will ultimately determine the rpm that this critical speed takes place and we get harmful vibrations and wobble or complete shaft failure. Increasing the diameter of the shaft, stiffness or reducing its length will almost always increase the shafts rpm where this harmful deflection takes place.
By lengthening the spool, the stiffest part of the assembly, it also shortens the driveshaft at the rear which raises the rpm at which that shaft reaches critical speed.
sorry, cooking Mother’s Day breakfast and replying at the same time
*edit to add more in depth info on this topic
https://en.m.wikipedia.org/wiki/Critical_speed
https://en.m.wikipedia.org/wiki/Rotordynamics
https://en.m.wikipedia.org/wiki/Torsional_vibration
Ok.. That I can understand
Short answer, Arrma designed the rear shaft far too long for any REAL speed application..!!