FIP is mis-named. It's not fielding-independent, otherwise the formula would contain an element that distinguishes, e.g., whether you have someone like Byron Buxton playing CF behind you. It's a number constructed from plate appearances decided by not putting the ball in the reach of fielders - HR, SO, BB+HPB. That would lead to a less catchy name; I don't have a better name to propose - "What Might Have Happened If The Ball Had Never Been Put In Play". Pointing to the convergence of FIP and ERA in the long-run is circular logic - FIP is constructed as a linear regression (of the above-named factors) to actual earned runs, and contains a seasonally-adjusted additive factor to line them up more or less exactly for a given season. It seems to have some small predictive power for future ERA, versus just ERA itself. But as a method to determine pitchers' actual seasonal results with defense factored out, it's pretty far from the mark. I wouldn't use it for a HoF argument.