Withdraw and Wallet hand-off fees

[jcnelson]

One aspect of the sBTC system that may surprise users is the fact that the Stackers need to pay a BTC fee each time they perform a wallet hand-off. That extra BTC has to come from somewhere, but who pays it?

The SIP currently calls for every sBTC holder to pay the fee, collectively, in the form of a pro rata deduction in their sBTC holdings. This would ensure that the total outstanding sBTC gets reset to whatever the new BTC balance is, minus the hand-off fee. However, users may find this surprising (and negative) because their sBTC holdings diminish simply by keeping the sBTC in the system. The BTC fee is a "decentralization fee" -- it's the cost of having a rotating set of signers (Stackers), which necessitates re-keying the BTC to the new signers, which in Bitcoin's case, necessitates converting all the old wallet UTXOs into new wallet UTXOs on the hand-off. Moreover, this fee is a function of the total number of UTXOs, which can be large and unpredictable.

What are some alternatives?

[AshtonStephens]

Closing as duplicate of stacks-sbtc/sbtc#323