On Sun, Dec 20, 2015 at 6:24 AM, Peter Todd via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> What I proprosed is that a consensus-critical maximum UTXO age be part
> of the protocol; UTXO's younger than that age are expected to be cached.
> For UTXO's older than that age, they can be dropped from the cache,
> however to spend them you are required to provide the proof, and that
> proof counts as blockchain space to account for the fact that they do
> need to be broadcast on the network.


Yes, this is almost what -has- to happen in the long term.

Ideally we should start having wallets generate those proofs now, and then
introduce the max-age as a second step as a planned hard fork a couple
years down the line.

However,
1) There is also the open question of "grandfathered" UTXOs - for those
wallets generated in 2009, buried in a landfill and then dug out 10 years
ago

2) This reverses the useful minimization attribute of HD wallets - "just
backup the seed"