"How can a notion of arbitrary tonality be used to support a notion of fixed tonality?"

BINGO!!

On pitch standards (current status of 440=A):    CLICKY #1.

Worth a read:    CLICKY #2.

Can you imagine what the economic impact is on musicians who own fixed-pitch instruments such as flutes, various other woodwinds, brass instruments—cathedral organs (!!)—et al, whenever some bureau of standards decides, for whatever reason, to change the pitch standard?

If you raise the pitch of most instruments, it tends to make them sound a bit brighter, but for the most part, it doesn't make all that much difference. Until your grand piano explodes!

And who might lobby for such a change? Musical instrument makers perhaps?

A piano can be retuned. I can retune my guitar (hoping the change doesn't alter stresses too much). But my upstairs neighbor has little choice but to toss his $9,000 oboe into the Dumpster and buy himself a new one. And St. Mark's Cathedral ten blocks north of where I live, with it's great monster Flentrop organ with its forest of pipes, from the size of a large tree-trunk down to the size of a piccolo, has a real problem!!

This might explain why, whenever someone decides to alter the pitch standard, mobs of musicians tend to storm them with torches and pitchforks.

The establishment of a pitch standard—AND—the development of even temperament amounted to little more than a minuscule compromise in tuning which allows fixed-pitch and variably tunable instrument to play together. And it also made possible such works as J. S. Bach's Twenty-Four Preludes and Fugues for Well-Tempered Clavier. One prelude and one fugue in each key, both major and minor, without having to stop and retune the instrument for each key. It was also noted early on that this made modulating into other keys possible, hence greatly enriching what composers could do. Made Beethoven, Tchaikovsky, Rimsky-Korsakov, Leonard Bernstein, George Gershwin possible. And, for good or ill, Shostakovich and Alben Berg.

And you know what? The vast majority of people, no matter what anyone tells you, don't even notice the difference between pure intonation and even-temperment.

Fact.

Don Firth