Twinks may be a minuscule portion of the sub base but we make up one of the highest income brackets of the sub base (how may we spend on WoW). Many Players that Twink spend absurd levels of money Twinking out their toon(s). This is also why Blizzard puts up with Prepared's shenanigans on Aegwynn (my first Realm) because he pays for 25 accounts monthly. People complain non stop about him in BG's but Blizz lets it slide. On the other hand, Twinks have been nerfed repeatedly over the years because of bads that can't be bothered to L2PvP and cry on forums to Blizz to nerf Twinks and/or remove us from BG's, which Blizz has done both.
Prime recent example is that Blizz is nerfing Felmouth (probably adding a level 90 requirement to it) all because of a few bads crying that low levels are using Felmouth to destroy them in duels. I can't tell you how many times I was reported for my 10 Hunter back in WotLK for "hacking or cheating" because I could destroy up to around level 20-23, 24+ (or Pallies) became an issue. I even had a Dev pst me one day assuring me they weren't going to ban me since I was doing nothing against ToS. This nerf goes back to a comment I made in various other threads, if you're too lazy to farm your own consumables, then you really shouldn't be Twinking to begin with.
The Currency was removed at the end of WotLK in prep for Cata because it didn't fit with Blizz's future plans. Was it because of cyrbaby 80's (at that time) unhappy that lowbies had WotLK Exalted rep, don't know because Blizz made no mention of that. But one thing is likely for sure, that they didn't remove an entire currency tier just because of players "abusing the system" with it. You may not have played back then but there weren't Challenge mode anything. The only thing any lowbie did that wasn't in Blizz's design was entering TotC & ICC, but that was due to Blizz's lazy design not an abuse of the system. The only other thing I can think of that low level Twinks have been found doing was the TBC Dungeon bags that were added in Cata. Ofc, this was only Level 15+ but they were getting away with equipping gear equivalent to character levels 60-70 at 15+. Blizz banned many an account for that practice.
Over the years Blizz has seemed to go out of their way to nerf low level Twinks (I've been Twinking since TBC, seen a ton of nerfs) which stops at around the 49 tier, I don't think they've ever cared too much about 59/60+ Twinks, except the 60 DK Twinks in WotLK, they got hit very hard, as did DK's in general. And over the coming xpac(s) Blizz will likely continue nerfing low level Twinks and it's 100% because of higher level Players complaining about us, period. Those people abusing the system back in MOP for Challenge mode were 90's, not low level Twinks.
