That is not reliable. Windows for example won't connect to an open AP that previously had encryption enabled. At least not automatically. The next problem you might face is that TLS enabled websites will trigger an error warning.
Your TLS argument is true. That however does not make it any less insecure.
You see, People around the world click 'proceed anyway' on so many of those websites. That is what happens when APs in coffee shops are misconfigured around the world.
It is barely a useful prompt (if at all).
And keep in mind this is a MITM we are talking about. He could simply, replace an instance of a website with his local version using 'http' instead of 'https'. The prompt would not even show up in this case.
You can blame it to users stupidity all you want, but if people click on "continue" it's their fault. They can read. If they can't, they shouldn't use a PC in the first place.
Not everyone has access or the funds to get a jammer. Making it more "Vulnerable"
Take a gander at EVIL TWINS: https://rootsh3ll.com/evil-twin-attack/