The principle behind the wealth by level table in the DMG is to summarize the amount of magical gear that a character will have access to at each level.
It's clear that the potions, scrolls and other consumables that are available should be included in the wealth count. Anything that has been consumed can't benefit the character and thus isn't counted as wealth.
Does this mean that characters are expected to use consumables regularly? What is the opportunity cost of using those consumables? Do they become free over the long term?
For example, a party that uses a Potion of Heroism for every character in every fight they can up to level 20 will get much more treasure over 20 levels than a party that saves their potions for the fights they need it.
Best Answer
The suggestion I'd make on handling parties that use an abnormal amount of consumables is the one of staying true to what little rules we do have: hand out an appropriate amount of treasure, and let the players decide how to apply it. If they spend lots of gold on consumables, they end up with fewer permanent magic items. If they spend little... well, they'll have more shinies. In other words, I do not suggest enforcing WBL on characters that already exist - and there is no rules premise for doing so, as far as I can tell. What you should be focusing on is handing out the correct amount of treasure.
Starwed's answer contains information on how much gold the game assumes characters will collect throughout their adventuring career.