Crowns are made to last for a long time. In fact, one review of published research conducted during 2007 showed an estimated 94% survival rate at 5 years and a 90% survival rate at 10 years. Those are encouraging figures, but it is possible that crowns will need to be replaced, and sometimes not even because of damage to the crown itself.
For example, it is possible for your gums to recede slightly. When this occurs, a gap can be left; debris and bacteria can infiltrate that space, creating an unsavoury smell and putting both the root structure and even the underlying bone in jeopardy.
If that should occur, you may be given the option to replace the crown or cover the gap with a filling. It's often easy to be tempted to go for a filling since this is less expensive than fitting a new crown, but there are plenty of reasons why you shouldn't.
The stronger solution is the better one
Fillings can be very strong, but they tend to do best when the filling material covers a very small amount of space, such as a crack or cavity within a tooth. When the material is needed to cover a relatively large gap, it isn't as able to maintain its strength. Crown placement is different. It completely encases a tooth, therefore reinforcing it in a way that fillings cannot.
Added failure points
Fillings are more likely to break than crowns when they are expected to cover a larger area. Compounding this issue is the fact that adding filling material around a crown is going to introduce more possible failure points. Even if both materials were equally as strong, it would make sense to go with a crown since there will be no line that can break between crown and filling. It always pays to go with the least risky route when it comes to your oral health, so one option working alone is going to be better than two working together.
Possible shrinkage over time
One thing that people don't understand about fillings is that they can actually shrink as time goes by. This is a process known among dentists as 'creep', and it's a common reason for general fillings to fail. Covering a gap created by an ill-fitted crown with a filling might work in the short-term, but, even if the filling doesn't crack, it might shrink to create a whole new gap. Better to get a new crown fitted.