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 in extracting maximum performance. Whilst the prime objective is obviously to facilitate the stylus tracing the groove and for the arm to track across the record, this is as meaningful as suggesting the prime purpose of a car is to get you from A to B - of course it is - but in a 1974 Ford Escort, a new Bugatti, a 4WD Land Rover or perhaps in the comfort of a Rolls Royce? There are many ways to achieve the goal of getting to ‘B’….cross country? Shortest time? In the joy of a classic? or with a chauffeur driving and a glass of champagne in hand?
In the case of tonearm design, much of the performance depends on mass/stiffness of the cantilever suspension. High performance Japanese cartridges tend to be heavy and stiff. And the stiffer / less compliant the cartridge, the more mechanical energy is transmitted/transferred into the body - and on through into the toneararm.
The musical signal generated and sent to the amplifier is NOT simply what the stylus sees as it traces the groove but a combination of what the stylus sees, plus the signal generated by the tonearm as it reacts to the energy input from the cartridge. The music you listen to through the speakers is a super-position of the two waveforms - what the stylus traces, plus how the tonearm reacts.
The stiffer the cantilever, the more kinetic energy is imparted to the arm and the more prone the arm is to generating coloration. Some frequencies will be accentuated, others blurred and often 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.
A tonearm must, therefore, exert significant control over the cartridge to negate this ‘counter-movement’ or ‘reaction’ to the inertia of the cartridge body as the latter tries to vibrate in sympathy with the stylus and cantilever.
In short, the tonearm must ensure the signal generated is only what the stylus sees in the groove.
Or to put another way.......... a tone(less)arm !!