I think if you make a small test you may discover that replacing a 732 by a bigger plane does not bring a very noticeable increase in profits. Plus, you loose "seniority" in routes with competition.
True, by replacing an aircraft for a newer one there may be a lot to be saved in maintenance costs. However, this is assuming you can actually flog the old plane on the market at a reasonable price. Not very likely in mature worlds.
Not to mention how long and how much work it is to close routes and reopen them with new planes... :cry:
This is just a personal opinion but, for what it's worth, I simply do not bother to replace aircraft in routes where I'm already offering at least 70 seats. Instead, I prefer to put new planes to use on new routes.