There are people with Sprout funds that came to Zcash because they care deeply about privacy. The only “procedure” that has ever allowed for Sprout migration (via turnstile) is completely unacceptable to some people as it requires the use of the transparent pool. To some Zcash will become a complete failure if shielded funds are ultimately lost forever for all users that refuse to use transparent Zcash.
The privacy preserving answer is below. It just needs to be built to avoid the loss of shielded funds/privacy loss choice where privacy purist users are still stuck with.