Edit: Fees with different block-sizes (assuming 100µBTC/kB)
1MB=100mBTC
20MB=2BTC
56MB=5.6BTC
Those figures imply the fees won't be be significant until the block reward drops to 12.5 or 6.25 BTC/block. Note that the 2BTC figure is almost 10% at the current block subsidy (but the block-size is limited).
Nice to see your estimates. I have long thought about where the reward/fee crossover point is. Obviously it depends upon rate of tx growth, but 30-60MB seems likely to be the block size range.
Agreed also that the block reward is still in the subsidize-to-grow-usage phase, tx fees are really useful just as an anti-spam measure. Beyond that, trying to force tx fees higher at this point only risks driving ecosystem business to alternative cryptocurrencies.