Expiring UTXOs might be a step too far, especially when other projects considered state rent, where you pay more fees when you try to spend an old UTXO. Of course some UTXOs may never be spent, but that only means they are effectively out of circulation, giving more value to the mining rewards, whether these rewards are subsidy or fees. So miners win whether these old UTXOs move or don't, and if they move, miners eventually get rewarded for the marginal cost of storing these UTXOs for so long.