You certainly can. What you can't do is patent and be 100% sure that it won't ever get invalidated in a challenge. But that never happens, because the incentive landscape makes it more attractive to licence: a company licencing instead of challenging passes the buck to its competitors, and even more importantly to potential future competitors. Patent troll companies are effectively offering moat-as-a-service and those moats perform just fine even if they are just imaginary moats.