One significant issue is the added layer of uncertainty this introduces regarding the finality of transactions. Particularly, if a transaction were to be included in the last block allowed by such a mechanism, it might encourage miners to instead attempt to reorganize (reorg) the blockchain to exclude that block in favor of mining a new one. This scenario undermines the stability and predictability of the network.

Additionally, the proposed expiration mechanism could be exploited to conduct bandwidth-wasting attacks. For instance, an attacker might publish a transaction slated for inclusion only in the next block but deliberately set the transaction fee below what would be considered attractive for miners. This maneuver could clutter the network with low-priority transactions, thereby reducing its efficiency.

A potential solution to expire transactions without disrupting network incentives involves creating a conflicting transaction. This transaction would be time-locked until the original transaction's expiration time but offer a higher transaction fee and feerate. Once this time-lock expires, miners would find the conflicting transaction more appealing to include in a block due to its higher fee, effectively rendering the original transaction invalid. This method aligns with the miners' incentive structure, as they are naturally inclined to prioritize transactions that yield higher fees.