Thank you! That makes perfect sense. Now I feel comfortable. I knew there was a maximum size, but I only thought of that as important for keeping an attacker from flooding off all the other clients.

[quote  author=DannyHamilton link=topic=148872.msg1580141#msg1580141 date=1362362258]Keep in mind that the block subsidy will continue for 136 more years, so you are getting concerned about something that probably won't happen in your lifetime.  Trying to predict what technology will be like and how people will deal with the complete loss of the mining subsidy right now is kind of absurd.  Can you imagine asking someone in 1877 about their predictions for technology in 2013?
[/quote]

But "block subsidy" will be very small long before then, right? I'm not sure /exactly/ when it will be so small that it can't support sufficient security on its own so it was easier to ask about when it is zero.

Hm. I guess I still don't understand.  

Fees should be very small, only the cost of including a transaction, which is almost zero because the transactions are only a few hundred bytes of data. Won't the mining difficulty keep going down so that the miners stay just-profitable?  Why assume that fees will go up instead of security going down?

Edit: I feel like if I just understood this one last thing I'd really be able to trust bitcoin.

I have been trying to understand Bitcoin and I've read the whitepaper and many messages... but I can't figure out why Bitcoin will have security once all the original Bitcoin are distributed.  Because there is no more free coins won't the difficulty be very low and so it will be cheap to rewrite the history?