Well, yes, but ElSupreme above mentioned retaining "Signature status" without a Signature car, which essentially amounts to access to the color, correct?
....
Still, you basically reiterated my point in your second paragraph. The Signature models were meant as a way to collect funds, a financial incentive. If you give away one of the two primary benefits of the Signature cars (the other being early delivery) to an owner in perpetuity, there's no incentive for them to buy a Signature model in the future. And Tesla wants you to buy more Signature models in the future.
Sig has amounted to almost nothing, but, if Tesla would "brand" all future purchases as signature, then I would appreciate that. Can't speak for others, of course.