When someone says their concern is "electability," continuing to emphasize how Bernie is the most progressive candidate can inadvertently have the opposite effect: it can reinforce the Establishment frame that Bernie is "too far left."
But these jokers don't have a leg to stand on. Their "safe bet" candidates lost every election for 20 years.
"Biden has a lot of experience."
"Buttigieg is well spoken."
"Klobuchar performed well in the last debate."
"Warren is my #2 pick."
Then talk about your concerns about their viability.
The trick is to help people to see that his policies—and his courage in standing up for working people—make him more attractive to more people (i.e., more viable).