Every expansion is like a reset button for equipment so new players can catch up. Achievements get easy or if not doable anymore turn into feats of strength. So Blizzard does everything to level the playfield so all players - new and old - have pretty much the same basis. EXCEPT for gold.
Sind gold is the currency since vanilla the only way to equalize was to give more and more gold with each expansion so new players can catch up to at least afford basic stuff. But the auction house increased the prices for even basic items until it's pretty much stupid now.
The solution - have a new currency for each expansion to pay for all stuff. So if you want to buy Legion gear you need to pay with Legion gold and you get legion gold in legion. When you go to BC or WotlK you get BC or WotlK gold. Just think about it for a while before flaming and dismissing the idea.