5 years is a very long time. What life changes may come along in 5 years that will impact what you want from your career, e.g. kids, new relationships, relocation, illness etc.
And what are the practicalities involved - you're not a slave, so they can't keep you from leaving. It is just a matter of what damages or penalties would apply. Will these be specified in the contract?
Why not get them to apply the carrot instead of the stick? Suggest they put in place a long-term incentive programme, i.e. a new bonus system (on top of any existing bonus system) which is spread over the next 5 years, on condition that you remain in their employment. Stock option schemes offer this kind of long-term incentive in many publically quoted (and some private) companies (provided that there is reasonable growth in the stock price).