The real problem with the idea is not the negligible advantage it gives but the precedent it sets, as someone said it is the thin end of the wedge.
If not having to pay upkeep on your carrier ever again wasn't an advantage why would you pay money for something that nobody at all in the game can see?
I buy some cosmetics for my ship they don't affect anything other than its appearance but at least I and others can see them.
Yes, on the consequence that it "create a precedent" that is highly likely. Although, as mentioned earlier, Frontier already has limited run items and services paid with real money; so I don't really see how worst could it be to implement an item that you buy once and avoid to pay upkeep.
Not sure about your question: I don't get a carrier because I want to show it to others; it is for my own benefit and for my game loop (exploration). The only advantage I get is to not be bothered to come up with a sum of money every X days; which translate in saving time to do a tedious operation that I would not commonly do, just for the sake of paying the upkeep.
In terms of gameplay, the upkeep is a relic of the old MMO where players were incentivized to log in to pay their upkeep. Nowadays instead of the negative reinforcement in MMOs (pay upkeep) there is the positive reinforcement (log in every day constantly to get better and better rewards); even old MMO have limited to no interest in using archaic features like upkeep for [something in game], and the new ones that offer that service are just doing it to be sure players log in, so you can track the number of recurring players more easily (since you know that they must log in at least once a month or whenever their upkeep is expiring).
As such, I would pay money for my own benefit; which would not affect neither the economy (the money for the upkeep are not going in the economy cycle from what I understand; there is no "universal vendor that repair carriers" in game, which would take those upkeep money) nor anyone else in the end.
Cosmetics to me are unappealing; except some decoration inside the ship (what I see 90% of the time I play is the interior, not the exterior)
In fact, cosmetics affects much more than what you think... The whole Fortnite and LOL monetization model is based on skins and cosmetic items. It is easy to dismiss a cosmetic because "it is just for show"; meanwhile kids get bullied if they do not have the latest skin and force parents to buy those skins with real money. I assume you have no kids, otherwise this should be pretty much a given. If it is not for money, it is for keeping you logging in; so you have a constant flow of players that you can chart and show around during investors calls.