Of course, maintenance costs is really the only reason to replace a loc (assuming a player does not want to change to a different loc for some reason). If maintenance costs never increased, there would be no incentive to ever replace a loc.
However, comparing the maintenance costs of an older loc to that of a new one is not the only consideration. Another important consideration, and many times an even more important one than comparing old versus new maintenance costs, is the price of a new loc.
Granted, the game usually makes the decision for me. That is, if I have the money to replace my older locs, I will replace them. If I don't have the money, I'm usually looking at going bankrupt. So the decision often isn't mine to make, but there are fairly rare occasions where I'm in a position to have to chose between spending money buying new locs, or spending money doing something else (usually expanding my network to prevent the AI opponents from grabbing nearby cities).
My point is, are you really saving money in the long run by replacing locs that are only 10-15 years old, considering the price of a new loc? Again, the correct answer probably lies in what type of scenario you're playing, although frankly, looking at the victory conditions of most scenarios and their relatively short duration, I'm doubtful whether loc maintenance costs plays a large role in winning or losing. I could very well be wrong. What I do know is that once I reach a certain point in sandbox mode, where that cashier's bells rings almost continuously, maintenance costs seem to mean very little. Of course, if I'm running 200 trains all over North America and were to let them all age to 50-75 years, I would probably start to feel the pinch, but I'm guessing the constant breakdowns that would happen would pretty much preclude such a scenario in any case.
I should mention here that I never play scenarios. I only play sandbox mode, usually until I've bought out all other companies and own the entire map. Even if it started out as a scenario, I never pay attention to the victory conditions and simply "Continue Playing". I do play the advanced settings, at maximum difficulty, and usually end up around 203% difficulty. That's my "sweet spot" which I enjoy playing.