I don't know. I consider a reasonable chance of success to be in the 75-90% range for the kind of things my character should be good at, with many things being automatic successes (generally not combat things, though). The Leverage crew is a good metric for how skilled PCs should be.The truth though is all game systems are designed around situations where the 'bonus' to a roll is significantly smaller than span of fortune available from the fortune mechanic. In D20 this means the design is built around situations where your bonus on the D20 check is rather smaller than 19, and probably not larger than 10 or so. Any game system starts breaking down when the fortune is a rather small input to adjudication or where the range between bonuses of participants becomes large. It's not just a D&D thing. It has to do with the mechanics of playing well together and having reasonable expectations of failure or success.
I am however OK with that success rate being achieved via some kind of meta-currency ("This is an important roll, and I'm not going to fail it.")