It's quite common that key persons(usually in management/business) stay as consultants for a while, like few months to few years, when they're leaving/boostrapping their own company. The company will have enough time to fill the position and train the new guy and the person leaving can keep stuff running and turn over accounts/information/contacts.

Try to negotiate somekind of new contract. It's good to get a solid contract, say for like 6months, which is renewable or terminated if both parties agree, so they can't drop you easily(you can have some income projections) but they cant keep you forever either, and you can terminate it if it doesn't work out.