So lets say this protocol is widely being used and quantum computers come along. No problem just square the difficulty level. But what if the difficulty is 2^70 or more before you do it? Now difficulty is 2^140. Whats happens when x in the fee table is less then one wie? You really need to extend the decimal or make sure x is min of one wie.
If you don't and fees disappear completely it may ruin the world economy if ether becomes a dominate currency. You could speculate for ever on what that would exactly look like but it would definitely not be good for the system.
It would take a galaxy sized computer with current tech to get that level. But yet I would not be surprised to see it get there even in my lifetime...