I have to say that you can't expect that you will always get to do the exciting work, unless you are in a rare field. Sometimes we have to do less than glamourous stuff.
Of course if you feel no loyalty to your employer you can bail and look for work more to your liking. But do that too many times and you will have a resume which doesn't reflect well on you.
But you also have to know yourself well enough to judge yourself to see if you have the right skills and attitude to make a go of this legacy project - if you won't be happy from the get go, you won't be productive and should think about your options.
James