There simply were not enough commodities at the top to spend influence on so there were significantly more players with essentially unlimited influence than there was a supply of such things. In the real world there's always something for a rich person to spend their money on.
Indeed the lack of influence sinks and the unbounded inflation that resulted was the primary cause for sky high prices for the very top items with limited supply. Lots of people liked to complain about the market without really understanding the dynamics of how economies work; if they did they would have realized that limited supply or 'ebil marketeers' weren't the problem, but rather
too much money existing. A few did realize, as evidenced by the various influence destruction projects.
In some games, you have to routinely repair your gear, or there are (necessary) consumables sold by NPC vendors, or various other methods of counterbalancing the wealth that gets created out of thin air every time an enemy is defeated. COH didn't really have that once you hit the level cap and no longer needed new SOs, you could just accumulate influence forever and never have to spend it. The 2 billion cap was the only limiting factor and required creative methods of storing influence past that.
Even the sometimes exorbitant prices of IO recipes didn't impact that, as if you bought a high end purple for a billion influence, only 100 million of that was removed from the economy. The other 900 million went to
another player, ready to bid on something else with. In an off-market trade for the Glad Armor unique, if someone paid 4 billion for it, all 4 billion went to another player and absolutely none of it was removed from the system.
Meanwhile, billions upon billions of influence was being created every day just from enemy defeats in regular missions, task forces, AE farms...
The real 'fix' for the COH economy would be to give rich players something to spend their money on that destroyed it in the process, preferably something repeatable that would keep them coming back for more. High-end vanity pets costing hundreds of millions of inf, inf-purchasable time limited buffs, that kind of thing.