first off a bernie presidency isn't going to be able to do it unilaterally. 2nd even if it did it won't happen immediately. 3rd even when it does happen there's going to be a transition, it's not going to be night and day. if medicare for all happens it may very well be like now where people (like my parents) they're elligible for medicare, and they have separate private insurance that supplements that coverage.
but the long term effect is that the next time a contract is up for negotiation the union isn't going to have spend their fistpounds on demanding healthcare, and management isn't going to be able to say "we're giving you such great insurance you gotta give on something else". health insurance won't be part of the total compensation package that is negotiated so if the value of that package were to stay the same, pay would be massively increased.