In general, the time spent in the leveling phase is so insignificant, that buying gear off the AH is pointless. You’d out level it far too fast for it to matter.
It’s one of the largest flaws of WoW, but kind of inevitable, when they had to stat squish like three times, and also kept adding levels. To remain accessible, the leveling process needs to be pretty fast.
Gold inflation is a different beast, and is ironically caused by blizzards efforts to make sure casuals have enough gold to afford repairs/consumables. It started with generous gold rewards on dailies in BC, and has continued with other infusions of gold that “just happen” through normal play.
If acquiring gold was a deliberate thing that you did, there would be much less inflation, but more poor players complaining about not being able to afford anything. This one isn’t really solvable, not without very heavy handed intervention on blizzards part.
To get back to the levels of vanilla or BC, they’d have to do something like the following:
Introduce two new coins:
Electrum, at a conversion ratio of 100 gold to 1 electrum.
Platinum, at a conversion ratio of 100 electrum to 1 platinum.
Rebalance mob currency drops and vendor prices to make electrum the new gold, but make repair costs tied to platinum. This would very rapidly devalue everyone’s hoarded currency, and effectively reset everyone’s wealth.
This will /never/ happen, because there would be endless screeching on the forums.