That is a lot of but's and if's We hope to see a increase in transaction volumes, but we need massive adoption for that kind of transaction volumes. I also think most pools will combine efforts to get those 50 Btc blocks and the smaller individual pools will be pushed out of mining.
Let's hope the adoption increase enough to surpass the block rewards, long before it reach zero.
a 50btc txfee total is at the moment 0.012btc a tx.. (50btc/4200tx), lets average it to 1200txs of fee paying users = 0.04btc (over $9/tx)
who would pay $9 a tx right now to give miners a 50btc tx reward if block solving reward stopped today.
even in the future of 20mb the average customer usage and voluntary addition of a fee (cos not all blocks are filled and not everyone wants to pay) would be 45c at current btc price.
even if bitcoin went to $1000 a coin when the 20mb block limit was in place and every byte of the limit was filled with tx data, and every tx had a fee..
50btc/84,000tx =0.00059524btc/tx = 60c/tx
i think 50btc tx fee reward per block at any fiat price is asking too much.. i think the bases of keeping bitcoin active and useful would be a 1c/tx fee..
which based on a full 20mb limit and all users paying a fee. and btc value being $1000 each would be a 0.84btc total reward... definitely not 50btc.