There would be a certain percentage of miners that would try and include any low fee paying transactions in their blocks (and maybe the community miners too, who want to provide this service) but for anybody with a reasonable share of the total hashing power it would definitely be against their long term self interest to move too far away from a cartel price (whether the cartel was natural or not) and I think that miners with a large share of hashing power would act according to these long term interests.
You could just as easily say that in the classic tragedy of the commons scenario, anyone with a reasonable number of cows would limit their grazing voluntarily because otherwise they'd be acting against their long term interest, so we don't have to worry about the tragedy of the commons. I think your argument fails for the same reason.
Ok, my main argument was that the tragedy of the commons wasn't a suitable analogy because the economic factors are different and you can' t really make a square peg fit a round hole by saying that the consumers are selling a 'willingness to pay resource' to the suppliers.
The fact of the matter is that in reality the tragedy of the commons often doesn't occur and in these situations people will self-regulate and act in their long term self interests and common grazing actually has happened historically very successfully. So even though it isn' t a suitable analogy in economic terms it might help in supporting my prediction of what would actually happen.
The market for digital products is similar in that once you have built the software for a computer game it doesnt cost you any more no matter how many games you produce. However we dont see the market price being driven down to the actual costs of production and below even though there is a willingness to pay a lower price.
This is a very dissimilar situation, because the game producer has a monopoly on the right to sell the game. If anyone off the street had a right to sell the game, you would see the price fall to almost zero.
Well you didn't quote me completely because I acknowledged that it wasn't a perfect analogy. The way in which this would support my argument is if, let's say, three spread sheet software suppliers were selling similar types of software to the same market. People might choose any one of the three products and there will always be a willingness to pay a lower price. However we don't necessarily see a 'race to the bottom' where the three suppliers continuously discount to grab these people who are only willing to pay the lower price. Even though in theory it doesn't cost them anything to get the extra income from these other customers since their production costs are fixed.
Miners who have invested a lot in hardware who's only income is from transaction fees would keep their fees at, maybe not exactly, but roughly the same prices. Most wallets would calculate fees at a price that would mean that transactions were mined by the majority or all of miners. i.e. A fee slightly higher than the current highest miner fee. This way all of the miners would actually get most of the transactions.
The smaller miners who were taking all the lower fee transactions might benefit but the bigger they became there would be more incentive to only accept higher fees as it would have more of an effect on the market.
These smaller miners also don't have the economy of scale and have higher block propagation times so their gains might be offset by other things.
Anyway this is just what I think would probably happen, I could be wrong.