Not "anyone", but it's all a matter of price. Given enough resources, all back-doors are shallow :-)
And when speaking of encryption, people should be concerned with crime syndicates or with industrial espionage, in other words entities that might have the resources to find and access those back-doors.
Seeing how cheaply the KGB compromised FBI and CIA agents, it's probably a price many can reach. You don't need to compromise the key, just fake evidence so as to put the target under suspicion and get it decrypted. They must often chase down false leads, so this might be rather unnoticeable.
And when speaking of encryption, people should be concerned with crime syndicates or with industrial espionage, in other words entities that might have the resources to find and access those back-doors.