@j-ed
Of course encryption is one important part of security. That prevents the message from being read by unauthorised people.
The other part of security is, to make sure that you and only you are talking to your counterpart and no one is listening what you are talking about. That is the job of TLS and it protects (it should do so) f.ex. ones email account from being missused. That is, what I want to get from my provider. Nothing more, but also not less.
You are right, that there is no way to guarantee that a mailserver uses a secured connection to the next one and so on. From this point of view email is always an unsecure communication.