Re: [bitcoin-dev] A BIP proposal for transactions that are 'cancellable'

[Gregory Maxwell <greg@xiph.org>]

Functionality such as this does not currently exist not because no one
thought of it before, but because it has been proposed many times
before and determined to be harmful.  The existing design of CLTV/CSV
were carefully constructed to make it impossible for a transaction to
go from valid to invalid based on the time. The most naive
construction-- e.g. push the current time/height on the stack-- would
have that property and was specifically avoided.

When a spend goes from valid to invalid it means that a reorganization
will destroy coins even completely absent any dishonest actions of the
coins prior owner in the coins recent casual history. Effectively a
coin with any kind of non-monotone validity event in its recent
history functions like a recently generated coin: a coin that reorgs
destroy. Bitcoin addresses the issue for recently generated coins by
not permitting their use for 100 blocks.  I've yet to see an argument
for a use case for non-monotone validity that still sounds useful once
the negative effects are addressed (e.g. by subjecting coins that have
gone through them to a maturity limitation).