I agree with this. I think Sanders and Beto are kind of similar in the following way. They're both perceived as likable -- and they are. But in both cases, I think their perceived likability is enhanced because of their most recent opponents. Beto's likability is decent to begin with, but it goes up about 50% when you put him next to Ted Cruz. Same thing with Bernie when you put him next to Hillary.I can't imagine the 18-29 folks continuing to vote for Sanders when Beto and Buttigeig become better known.
Young people like Bernie right now because they're still comparing him in their minds to Hillary. When they start comparing him to Beto or Buttigieg or some of the other candidates instead, he won't get that same magic bump any longer. (Same when people start comparing Beto to Sanders or Harris instead of to Cruz.)
Hopefully Trump will find things more difficult in 2020 for the same reason.