Think of it this way, if you are going to start a business you have to have a store front/factory/facility, right? So you go out and lease such a facility. That facility means you incur certain costs if you sell 1 item of 1 million. Now, in the long run you can get a bigger (or smaller) facility, so in the long run these costs are also variable, but in the short run they are fixed.
Fixed costs are due to various impediments like contracts, transaction costs, and the like. These prevent the firm (or consumer) from moving directly to the optimal position immediately. With the facility example it is somewhat obvious; youāll have a lease and youāll have likely paid at least 1 months worth of rent if not more. Youāll pay this often before you even sell a single item.
Server costs are fixed. CCP cannot run out and just get a new server on a whim. There are all sorts of things restricting them from doing this.
As such, the notion that a skin is preferred in that it does not entail additional server costs is just not relevant as those server costs are going to be incurred even if CCP sells no PLEX and has just 1 customer logging in. Granted with just 1 customer logging in they may have to shut down soon, but the point is they still incur those costs.
Fixed costs do not vary with output (however you measure it). It is literally a fixed dollar amount, say $500,000. You pay that. No more, no less. If one player logs in you pay $500,000. If 50,000 log in you pay $500,000.
If you sell 0 skins you pay $500,000 in server costs. If you sell 10,000 skins you pay $500,000. If 10,000 player sub via PLEX and 20,000 sub via credit card you incur $500,000. Same if the numbers flip flop (20,000 sub via PLEX/10,000 via credit card).
You can only not incur that $500,000 by shutting down.