This has nothing to do with efficiency. Costs do not imply inefficiency. You must incur costs to do something. If you want to make tennis shoes, you must incur costs to do so.
And no, inactivity is not the issue. The player clearly felt having an additional account was worth while. This too demonstrates opportunity cost. Not only does the account cost $15/month, but there is also the next best option as to what that $15 could be spent on.
Again, this is the definition of opportunity cost,
In microeconomic theory, the opportunity cost, also known as alternative cost, is the value (not a benefit) of the choice of a best alternative cost while making a decision. A choice needs to be made between several mutually exclusive alternatives; assuming the best choice is made, it is the “cost” incurred by not enjoying the benefit that would have been had by taking the second best available choice.
In other words, if there is a choice, then there is opportunity cost. If there is no choice at all, then there is no opportunity cost. And in this issue there are always choices.
- Do I get an alt account or accounts? This means opportunity cost is present because I could have spent the money I spend on alt accounts elsewhere.
- Do I train just one alt on the account(s). If yes, then there is opportunity cost, the forgone net benefits of training more than one alt.
- Is one of those alts a cloaky camper alt? If yes, then the opportunity cost is whatever I could have used on that slot.
- Do I use my alts for cloaky camping, if yes then the opportunity cost is whatever benefit I could have derived if I used on of the other alts.
Do you play Eve at a given time or do something else, if you decide to play Eve, then your opportunity cost is the benefit of the next best option for your time.
Most people do not even consciously think about this kind of thing when making a decision, but the notion of opportunity cost is what allows us to put it into a cost-benefit framework. It also allows us to evaluate various issues like the broken window scenario back upstream. We can use it to evaluate various policy proposals. A common refrain is policy X will create N jobs! But the next question is, okay, but how many jobs could that money have created without this policy? If it is M and M > N then X is actually a bad policy. Opportunity cost lets us evaluate the proposals for things like protective tariffs. Will it “save” jobs in industry Y? Maybe, but it will also mean higher prices for the goods produced by industry Y. That means everyone buying those goods will have less money to spend on other things reducing employment in industries not-Y. So we see that free(er) trade has cost N-hundreds of thousands of manufacturing jobs, but that completely ignores how many jobs were created elsewhere. And when confronted with this kind of argument the proponent of protectionism dismisses it by saying, “Those jobs sucked anyways,” without even knowing what those jobs are.
A failure to grasp and appreciate opportunity cost is not a good thing. And you have failed to grasp it. It applies anywhere there is a decision…a choice. That a person chooses A over B does not mean there is no opportunity cost it means that the (net) benefit to A exceeds the opportunity cost of B.