Paypal transaction volume is ~ 2.2 billion tx annually (VISA is ~ 78 billion and all electronic tx is probably on the order of a trillion tx annually).
Still lets consider a 30 year goal of 40% the size of Paypal =~ 1 billion tx annually. That is ~ 20K transactions per block.
ACH fees are ~ $0.20 per tx. So 20K tx * $0.20 ea = $4,000 per block block reward. Still I doubt fees would need to be that high. At ~$0.05 (whatever that ends up being in BTC) per tx avg block reward would be ~$1000 which would support a network 5x as strong as the current one.