If it happens, then my prediction is that it will be when there is less than a 75% chance of getting a 1kb or smaller transaction with inputs totaling more than 0.1 BTC included in the next block with a fee of 2%.*
*This is a completely arbitrary and unsubstantiated claim that I've made up on the spot simply because it sounded good to me.
I guess (hope) you mean more or less a flat rate which would be around 2% on a .1 BTC transaction?
If so, I've always thought that Bitcoin's best hope is to become something of a 'reserve currency' in which largish base transactions can occur. So it is not something I'd be particularly opposed to as one path forward for Bitcoin.
If not, and a 2% transaction fee is in the ballpark of what people are gunning for, I believe it would pretty rapidly suck the life out of the economic actors who are not capable of providing actual infrastructure support...and the barrier to entry here is rapidly making this non-tenable for most. Such a solution would be no more desirable than current mainstream financial institutions to my way of thinking.