I mean, these are optional cosmetics which have no bearing on gameplay so SE are in their rights to charge however much they seem is optimal so as to gain the maximum profits achievable.

Charge too high and less people will buy, denting profits, on the other hand charging too low also means losing out on profits if the buyers are going to purchase the item at a higher price anyway. Not to mention that with inflation, prices are going to increase as has happened with retail game prices. In fact, it's a miracle that the subscription cost has remained at $14.99 for 10 years after so long. If it was me, I would have increased the subscription cost to 20.99 by now to account for inflation.