For one, if someone takes ownership or control of the commons, they are not the commons any more.
Indeed, having a "commons" is the problem in the first place: no commons, no tragedy of the commons.
Bitcoin has no commons. Every node is owned by someone and they control it, therefore no commons, therefore no tragedy.
I say you are missing the forest for the trees. The entire Bitcoin economy and ecosystem forms a very viable and very real 'commons'.
For two, it is very much the case that the 'tragedy' occurs absent a mechanism to prevent it. 'forcible ban' or otherwise. You seem utterly backward on the whole 'tragedy of the commons' principle. To me.
That's good, because most economic reasoning seems exactly backwards from common sense.
If land is not owned by anybody, although legal formalism may call it public property, it is utilized without any regard to the disadvantages resulting. Those who are in a position to appropriate to themselves the returns — lumber and game of the forests, fish of the water areas, and mineral deposits of the subsoil — do not bother about the later effects of their mode of exploitation. For them the erosion of the soil, the depletion of the exhaustible resources and other impairments of the future utilization are external costs not entering into their calculation of input and output. They cut down the trees without any regard for fresh shoots or reforestation. In hunting and fishing they do not shrink from methods preventing the repopulation of the hunting and fishing grounds.
This can only happen when there is a commons ("public property"), which can only happen when there is a ban on ownership. There is no ban on ownership of Bitcoin nodes; each node operator owns his or her node and has control over it. No commons for there to be a tragedy in.
In Libertarian fantasy-land it sounds good, but in the real world there are countless examples of unmitigated disasters when the commons are divied up (or simply taken by those with the means to do so) and extracted out of existence.
One of the first which comes to mind is the salmon fishing in Bristol Bay which I participated in for many years. You won't find a bigger set of right wingers and libertarians than commercial fishermen, but every one of the recognized the value of treating the resource in a sustainable and managed way. Especially those who had been around in the bad old days.
For three, a lot of us, and especially myself, are pretty leery about individuals 'taking ownership or control' over the certain aspects of Bitcoin's function. Indeed, the difficulty of doing this is a major 'selling point' of the Bitcoin solution. In part because of the ill effects of such control in other solutions.
Only the nodes are owned, not "Bitcoin" (the system).
When only .0001 percent of the community has the realistic ability to operate a node (much less a mining setup) the system will be for all intents and purpose operated at the pleasure of a very small group of very probably like-minded people.
"People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public, or in some contrivance to raise prices. It is impossible indeed to prevent such meetings, by any law which either could be executed, or would be consistent with liberty or justice. But though the law cannot hinder people of the same trade from sometimes assembling together, it ought to do nothing to facilitate such assemblies; much less to render them necessary." - Adam Smith
I the open source world of Bitcoin, the protocol is the law. And the users are those who enforce the law. To the extent that it is practical at least.