You think today it spends too much energy. But this is another topic.
Let's say when rewards drop to 0 the fee will be 1$ per average transaction and we would be able to fit 5000 transactions in a block so there is 5000$ of fee in a block.
So If you want to trust a transaction that spends 10 000$ you should wait 2 blocks.
If you have a transaction of 100 000$ which is a lot you have to wait 20 blocks, which is like 3.5 hours, which is not a lot.
And even half of that because the fee you will offer needs to be at least twice the block reward.
But unrelated: in btc the electricity cost of transaction is too high. I did a calculation in one question here. It came out that the fees should be like 10$ per transaction.
From this it means that transactions should be at least 1000$ big to have 1% fee, making btc unsuitable for small transactions. But from your attack it means that the fee should be comparable with the transaction value making btc only reasonable for transactions of less than a dollar.