The sender is apprehensive that the new proposal might allow an attacker to redirect not just their own previously spent UTXOs (Unspent Transaction Outputs), but all funds that have reached a certain maturity level — indicated by the revelation of the previous preimage in the hash chain.

In traditional double-spending scenarios, attackers are limited to attempting to reverse their transactions. However, under the proposed system, there's a possibility that the attack surface could broaden significantly. Once the funds have matured past a certain number of blocks, as per the proposal, they could potentially be vulnerable to redirection by an attacker.

This effectively means that the number of blocks required for funds to mature acts as an "embargo period." Therefore, any coin that anyone spends within the interval between the fork point and the required maturity depth could be at risk. This suggests that the proposed model might inadvertently offer an exploitable avenue for attackers to perform more extensive double-spending attacks than currently possible.