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"