Given the gulf in finances between them and Celtic, Rangers would need a top manager to have any hope of closing the gap. In fact, if Rangers were to live within their means, i.e. break even on an annual basis without taking out further soft loans, they might not be second either. Imagine ditching two or three million quids worth of salary from the current Rangers squad and see what is left.
Short of some black swan catastrophe (or miracle, delete as appropriate), I can't see Celtic being knocked off their perch for the foreseeable future. They might screw up once every ten years but it has been shown that there's a strong correlation between the cost of a squad and its final league position (for example https://tomkinstimes.com/2014/01/money-buys-success-the-proof-you-need/). Someone would need to break Celtic's stranglehold on CL money to have any chance of shifting the balance, and that would have to be more than a one-off, too.
Going back to your point, I wonder how Rangers support will hold up if they only manage the odd win against Celtic. Will that be enough to keep the myth of 'closing the gap' going?