Good explanation, Highlandman. Makes sense.

But what it boils down to, really, is essentially the Tonic chord.

In a longer piece of music, it's possible for the piece to modulate all over the place (many long symphonic pieces do), but after spending some time in a new key, the ear gets used to that key, and the Tonic of that key becomes "tonal center" and can leave the ear satisfied if it resolves to that chord. Although most longer symphonic pieces eventually go back to the original key and resolve to the Tonic there.

The same can be said for lengthy jazz improvisations.

A long piece, then, can have several "tonal centers," depending on what the ear gets used to.

But the point should be made that this, in no way, calls for any "dinking around," altering the present completely functional system of key signatures.

Don Firth