I'd argue that inflation merely lowers the value of gold to be somewhat more in line with production. Production is simply not balanced the way the game is right now, it's quite hard to come by, it only increases very slowly with population growth, and by putting hundreds of cogs into buildings, while the main investment production is used for, districts, skyrockets in cost by mid-game as you rapidly tech to the modern era. Gold income on the other hand keeps much better pace with techs and civics, by mid-game it usually costs 20+ turns for a unit or building in most of your cities that you can still buy with a few turns worth of gold. Sure there's a cap on production costs and none on gold, but the game doesn't go on forever, it lasts 300 or so turns at most, production costs increase very rapidly in the mid game and by the time your empire's production catches up and gold costs start to really get out of hand, the game is almost over, if every game took 500 turns, it'd be much more of an issue. As far as actual usefulness, yes you need districts but wonders are not very good and as people have pointed out, the flexibility to buy units and buildings where you need them and to buy out the great people you really want is IMO more important.