Incorrect. There is a case in the Dukenminer casebook that says the opposite, a contract provision that allows termination by only one party at will IS enforceable. And that makes sense, termination clauses in contracts (leases) do not make a contract unenforceable, as an illusory promise. A contract provision can say pretty much whatever the hell you'd like it to and is 99.999% enforceable, although the common law tenancy at will does comport with what y'all are describing.