Dgad, "why not 14 inches tonarms"? Because of the lack of available space on most turntables AND because the decrease in tangential error relatively becomes less and less with increased effective length.
14" and 16" tonearms were invented for and mounted on special broadcast turntables/plinth to be able to play transcription discs of large diameter common in the 1940ies and 1950ies.
On the better calculated 12" tonearms the maximum tangential error is already between 1.3° and 0.3°. This is a relatively large advantage to the 2.8° to 1.6° in 9" pivot tonearms. Lofgren A,B,C (vs in between...), Baerwald, Stevenson or any other calculation can be applied to 9", 10" or 12" effective length. It will not alter in any way the inherent advantage in tangential error (read: smaller...) of the longer version.
For those not to be convinced by physics and logic, there is always the opportunity to compare 9" to 12" versions of the very same tonearm.
Which leads us back to Graham and the 12" Phantom II.
Wider soundstage, more stable individual voices in complex choruses, more free and detailed high frequencies and a kind of "relaxed feel" in dynamic scale music.