I do agree there has to be some kind of a gold sink, but not simply draining gold from players. Character gold cap is also not an effective counter because most of the people I know have, let's say, 5-6 characters on 2-3 accounts, but there may be more.
One more thing to note: WoW is (like almost all MMORPGs) a content based came.
Blizz, however, releases an expansion every 1-2-3 years, where the problems with inflation (a.k.a. amassing vast amounts of resources / equipment / gold) is dealt with naturally. Almost all of the materials or items from the outdated "old" content become just a means of leveling. The newly introduced prices of items are in sync with the expectations of a current pre-expansion endgame character.
(As, for example, entering Northrend for the first time and obtaining a few quest rewards grants the player a set of equipment, and that set is far superior than most of the dungeon sets or even raid sets pre-WotLK. Moving on to Cataclysm, a WotLK endgame geared raider finds simple quest rewards that outweigh his hard earned raid sets in performance, etc. TBC, MoP, WoD are no exception.)
So inflation influences the content. Therefore, inflation is countered by new content.
And that is irrelevant for a realm that will (hopefully) keep being WotLK realm for as long as it has players to play it. So AH prices will grow anyway, and that is a result of the realm being played. The longer the average player stays and plays in the same realm, the higher the inflation will be.
TL:DR:
Inflation also means players obtaining better gear, not just gold. Increasing requirements for joining a raid or a guild are also a kind of inflation. The better the average player becomes, the higher the inflation.