When an organisation has done the job it was set up to do, stop. Just because you exist (and employ people and do good work) does not give you a right to continue to exist.
You shouldn’t have to justify your existence: your founding aims should tell anyone that you have a right and purpose for your existence – IF indeed you still do have the same purpose. Having to change your overall aims is a sign that your time has gone: take the brave step of shutting up shop, rather than risk carrying your inevitable baggage on into a new direction. If the new direction seems so important, it will be met by a new organisation – perhaps involving some of the same people. Hopefully, if you’ve done your job right, the lessons you’ve learned will have been disseminated far and wide and so won’t be lost.