Neat idea!
This idea seems pretty similar to PRINCE — Privacy Incentives for ZEC, except applied to the Sprout pool instead of to the transparent pool. In general, I don’t see why the Zcash community should pay more costs and risks to maintain old UTXOs than they do for old shielded pools, which is why I bundled the two together in my proposal “charge higher fees for spending old UTXOs and Notes”.
To put that in a stronger and more positive way, what if the Zcash community signaled to all future users that we’re not going to support taddresses indefinitely, and we’re not going to support old shielded pools like the Sprout pool indefinitely but at least you can be assured that we won’t discriminate between one or the other. If you move your money into ZEC today, you can choose to put it into the t-pool or into the current z-pool with a similar level of expectation that the Zcash community will accept it and support it for a similar number of years into the future.
In contrast, if we continue to impose limitations and costs onto the Sprout pool (the first z-pool), but we don’t impose similar limitations and costs onto the t-pool, then that will signal to people that if they want to move their money into ZEC for long-term holding, that they had better move it into the t-pool.
I think there is a “social layer of governance” dynamic here that a lot of people are underestimating — I suspect a lot of people in these threads imagine that if the ZCAP, the Zcash Foundation, the Electric Coin Co, and 99% of people on this forum, all decide to implement some new policy, then all Zcash users will accept it and it will become the policy of the blockchain in the next future Network Upgrade. I really doubt that’s the way that it would play out!
I guess that if all of the above decided to institute a “hard/brittle” new policy like “taddresses just don’t work any more”, that there would be chain-fork and that the vast majority of the value and the usage would remain on the status quo fork of chain — the one that continued to provide working t-addresses. Those users like using t-addresses, they have their reasons, and we can’t force them to stop using t-addresses even if we wanted to. Nobody can!
On the other hand, if we instituted a “soft/incremental” new policy like “taddresses start paying 1% per year fee to reimburse the rest of the users for the costs of storage, technical debt, and excessive transparency”, I guess that it would go through — that literally everybody would upgrade to that new consensus rules set and the status quo chain would simply stop getting new blocks and new usage and would simply come to an end, just like what happened with the Overwinter, Sapling, Blossom, and Heartwood forks [*].
Your fun thought experiment above, and the related concept in PRINCE, could potentially improve that calculus by motivating a lot of users to support the new policy since they would see it as a more direct benefit to them and to other proactive, socially-helpful users.
[*] You might wonder why I don’t include Canopy in that list. Because that didn’t happen with Canopy! With Overwinter, Sapling, Blossom, and Heartwood that’s what happened — everyone chose to go with the new chain-fork with the new rules, and nobody chose to keep using the alternative. With Canopy there is a chain that people are using that keeps the alternative rules — Ycash. (Technically Ycash forked before some of the other Zcash Network Upgrades, just like technically Bitcoin Cash/BCH chain-forked from Bitcoin/BTC before the block size conflict was finally resolved, but politically, that doesn’t matter. Overwinter, Sapling, Blossom, and Heartwood were “uncontested forks” in which literally everyone upgraded at the same time as each other, just like the network upgrades that have happened on Ethereum. Canopy was, in contrast, a Friendly Fork event in which the blockchain split into two blockchains with a shared history, and now everybody can use either one or both.)