Most toilet problems can be fixed, it must be remembered that there are some UNFIXABLE toilet problems.
A toilet bowl that has an object (comb, toothbrush, toy, etc.) stuck in the passageway that can't be reached or dislodged from inside the bowl or from the bottom of the toilet will have to be replaced.
When the passageway in a toilet bowl becomes encrusted with scale, it restricts the flow. It's almost impossible to completely remove the scale, and it's time for a new toilet.
A cracked toilet tank can't be fixed and should be replaced as soon as it is noticed whether or not it is leaking. I had a student tell a sad story in class of knowing her toilet tank had a crack in it, but because it didn't leak she didn't do anything about it. One evening, she came home from work to find water cascading down the staircase from the second floor. The tank crack had opened up during the day when no one was home and a part of the tank fell off. The water drained out of the tank, and the water turned on. The water ran for most of the day.