One of the positive things about Failgold is it's more complex than Wealth. It takes a little bit of planning because you can only build one Wonder at a time, you can use resources, whip overflow, manage build ques, chop trees, swap civics, tech research order.... with Wealth you just research Currency and build Wealth. I like that Failgold has some strategy to it, I just don't like the historical implications of actively trying to not build Wonders.
By no means would I want Civ IV to get simple and become Civ V but if I was changing it, I'd nerf Failgold from World Wonders and completely remove it from National Wonders. At the same time I'd look at buffing build Wealth, Research and Culture with options to make them more rewarding and complex as the game goes on. Example: Wealth goes through multipliers like Markets again, or Wealth builds 10% better for each luxury resource.