The tragedy of the commons relates to unregulated use of common resources.
I dont think this is any way relevant. Mining resources are not under common ownership and there is no common right to use these resources. They are generally owned by private entities trying to make a commercial profit and you have no right to make them mine your transaction. Miners can choose what transactions they mine so the use of this resource is regulated by the miners.
Miners will simply set prices at what it is worth for them to mine and make a reasonable profit. With no block size limit they will be able to set the fee at which they can turn a profit at a lower level.
economic theory says that in a competitive market, supply, demand, and price will find an equilibrium where the price is equal to the marginal cost to suppliers plus some net income (because suppliers can always choose to do something more profitable with their time or money)
(Gavins blog)
Maybe a small percentage of mining will be paid for by community organisations or charity (but this still has to be paid for somehow) and people who are willing to wait longer can move coins for free but if you want a quicker service you pay a fee. So maybe these community miners will mine huge blocks (and be at a disadvantage due to block propagation) but they will be mining everybodys transactions. So the choice is a free transaction that is only mined by 5% of miners or the higher fee you pay the more miners try to mine it. You get a better service if you pay.
There are valid arguments for block size limits, such as, spamming and blockchain growth but I dont think tragedy of the commons is one of them.
The real danger is tragedy of the anticommons.
a type of coordination breakdown, in which a single resource has numerous rightsholders who prevent others from using it, frustrating what would be a socially desirable outcome
(wikipedia)
As suggested previously a large wallet might team with a large (say 30%) mining pool and charge their wallet users fixed transaction fees and pay the miners out of band. The mining pool is contracted to refuse to mine anyone elses transactions. Another competing wallet buys a large portion of the remaining hash power (say 25%) and now nobody has access to all of the hashing power for their transactions.