You're acting like Gore isn't the established vet & Mack isn't a rookie who tries to bounce EVERY run outside & has been stuffed for no yards gained or lost yardage on 44% of his runs. Indy's line shares some of that blame, but Gore has only been stuffed like that 25% of the time (on 3x the carries). Sure, every once in a while, Mack can bust a big one, but most coaches want a RB who won't put them in negative down/distance plays.
You're also ignoring the fact that Mack has played 3 games and sucked in two of them. 2.4 YPC in week 1, and -2.0 YPC in week 2. Yeah, he had 3 big runs this last week, but he hasn't performed "every chance he got" & outperformed Gore all season. He had a great game, but it was one game & until he learns to run plays as they are designed, earns the coaches trust in protecting the QB (especially when Luck gets back) & becomes more consistent, I doubt he will become the lead back.
There is obviously a better than zero % chance that Mack will become the lead back in a 60/40 split. But there has been NOTHING to suggest that this is going to happen anytime soon, and just because you want it to happen doesn't mean it will. There is a better than zero % chance that I will win the lottery, but I'm going out to buy a yacht based on those long odds. I might buy a ticket & see what happens, but it would be foolish to make plans for that money right now, just as it's foolish to suggest that Mack could easily take the lead job in Ind.