So... I start from "more transactions == more success"
I strongly feel that we shouldn't aim for Bitcoin topping out as a "high power money" system that can process only 7 transactions per second.
If bitcoin operated at the scale of PayPal, ~87 transactions per second, that would require roughly 20MB blocks. That's large, but not unmanageable.
People paid PayPal US$ 5.6 billion in fees last year. There's no way the bitcoin network is going to cost that much.