I’m late to this thread – missed it originally and just came to it via Keith’s posting. But would just like to reiterate what Laurence said. Isn’t the concern about PLEs being isolating dealt with by emphasizing the need for them to have a PLN component?
But I should also add that I agree with you and others who have said that looking at PLE(N) vs LMS as a dichotomy creates a false one for two reasons. Firstly, there is no reason that an LMS may not from time to time be part of my PLE, and secondly, even for instructors in an institution which mandates an LMS for administrative purposes (like access control and grade submission) it is easy to “escape” the LMS in any particular course by sending the students off to explore other networks and just using the LMS to collect back the reports of their travels.