Honestly i dont mind the banners, as many others as soon i get one i place them where i am, or well i place them in my arena. probably people get annoyed with them because invasion events but really isnt a big thing.
However i though banners have a function like everytime i killed for example: 50 enemies = banner lvl 1, 100 enemies = banner upgraded to lvl 2 and so on to the point the banner would be max upgraded. and honestly i would have liked that option because its an incentive to keep killing enemies to upgrade yor banners.
If people get mad with that, why not make:
1) your first banner from the 50th enemy make a banner NPC appear in your town if you have room for him/her, who would sell the banners from enemies you have defeated previously and got their banners (in case you want to place more for decorative purposes or buffs defense/arenas Zones from them) people dont keep getting banners from enemies only their first banners from the 50th enemy, and if they want more they can buy it from the NPC for a cheap price (copper)
2) to not make the "enemy counter useless" why no upgrade banners? make the first banner you get from the enemy have a low buff, im not good with that aspect but let say the lvl 1 slime banner give 2% more damage agaisnt slimes and defense from slimes, when players kills their 100th slime, they dont get a banner, instead their slime banner buff get upgraded to lvl 2 and give 4% more damage and defense agaisnt slimes, this would be a good incentive to keep killing enemies to upgrade your areas of defense, doing double damage and get half of the damage from the enemy you have the banner should be the max upgraded, and i honestly dont find that will be op because in order to get the buff you must place the banner, and only cover certain space, so if someone want to get the buff in all of their world they have to work first to upgrade their banner and then place them all over there. and once a banner its fully upgraded cant reach anymore buffs but will keep the track of how many enemies you have defeated.
3) i think the rooms for selling items from NPC isnt enough to cover the amount of banners from all the enemies in terraria and i honestly dont know if it possible to make "pages" for the NPC inventory, but if isnt possible, then maybe the NPC will sell certain set of banners every day, rotating the types of banners every day so players actually can keep buying those certain day if they need them.
that way i feel people would be happy in both sides, because they get their first banners will be only for the first time to make the NPC appear, people who want more banners can buy it for cheap from the NPC, and people actually have to work to upgrade their banners as incentive to get better buffs.