The design problem.
It is simple enough to design a platform at one end of a tube on which to mount a cartridge, construct a set of bearings, affix a counterweight to balance the other end, and call it a tonearm.
This might physically work, but it’s like comparing a model T Ford with a new Ferrari, and shows no real understanding of the physics involved. Apart from the obvious requirement for the arm to follow the groove across the record surface, a tonearm must exert a significant degree of control over the movement of the cartridge body as the latter tries to vibrate in sympathy with the stylus and cantilever.
As these two parts vibrate, they impart kinetic energy through the body of the cartridge and into the arm. As the application of a force requires an equal and opposite reaction, it follows that the whole tonearm must try to react by resonating, or vibrating about the bearings.
The musical signal is generated by the relative motion of the cantilever with respect to the coils and magnets inside the cartridge. The stiffer the cantilever mounting, the more kinetic energy is imparted, and the more prone the arm is to move in sympathy.
In a perfect situation, the body of the cartridge would be bolted rigidly to a heavy mechanical structure, so the signal generated is purely that of the stylus in the groove. But the system is not perfect. The arm is pivoted, and ( to some degree), will always vibrate in reaction to the stylus. This secondary, non musical, component is `added’ to the primary signal waveform.
In other words, the signal sent to the amplifier is a compound of both the music and any extraneous vibration from the tonearm.
This constitutes colouration, disguising musical information....... Accentuating some frequencies, blurring others and lingering beyond the point where the notes in the music stopped. Subtle background information is lost from the midrange, the treble can become harsh and bass either lost or accentuated. Accuracy of stereo imaging is lost, transients are diminished and front-to-back depth is limited.
In short, the tonearm must be designed to ensure the musical signal generated, is only what the stylus sees in the groove !
Or to put another way.......... a tone(less)arm !!