The first rationalization might take place at the economic level. A water-saving model could easily pay for itself in a few years and then, there is the good feeling of participating in the conservation of our natural resources.
Having to plunge a toilet more than once a week could motivate a homeowner to spend money on a replacement especially, if having made repairs to the flapper and fill valve didn't solve the issue.
Maybe your existing toilet has ugly scratches that make it difficult to clean. Maybe there are cracks in the tank or bowl that you're concerned will develop into a leak at the worst possible time.
The average cost to replace a toilet is around $400 with models ranging more and less based on the features and brands. Round toilet bowls tend to take up less room, are less expensive and better suited for children. Elongated bowls generally take more room, have more powerful flushing action, more comfortable, more stylish and cost more.
Replacing the shut-off valve for the toilet could be a good thing to do while you're replacing the toilet. Generally, it is as old as the toilet and having a reliable valve that works could be very convenient in a future repair or emergency.
There are a variety of videos on YouTube that could give you the confidence to do it yourself or simply, to have a better understanding of the scope of the project