If buying cosmetic items would be included in order to fund further development of Bannerlord I could accept it, but I generally don't like it very much, especially if new items are added over time. I fear that in order to attract customers these items would be of ever more grandiose designs (judging by other games that have this, e.g. Assassins Creed), and shift the visuals of the game over time as more people have them. It would be important to keep all items in harmony with the somewhat authentic medieval concept. In singleplayer it's different, as everyone can choose themselves, what to buy and include in their game.