Because even just 0.1%/year of $1 trillion is still $1 billion, or $20k/block. That's a lot more than you could easily raise by donations. And if it's consistent (which tx fees are *not*), it's probably enough incentive to keep mining moving forwards consistently.

...and that's a key point: sufficent tail emission or demurrage can fix the reorg problem where big miners frequently have incentives to play games reorganizing blocks. Fees alone simply can't do that all the time, as they'll inevitably be inconsistent.

petertodd

Counter argument I have heard before is: why would miners be more incentivized by a tiny security fee? The fee is "so small it won't significantly increase the supply", but then how would it increase the incentive to mine if it's so small that it doesn't really make a big difference.

badabing

bitcoin

I'm Peter Todd, cryptochronomancer/web-π dev, AMA

It's a tragedy of the commons. Relying on that kind of altruism is much more risky than just having a small security fee.