I think it's great that Susan Sarandon and Sarah Silverman plan on voting for Bernie. I loved Sarandon in Little Women two decades ago, and I got to see Silverman's standup at Oddball Comedy Fest a couple summers back, and she was fucking hilarious.
That said, this whole "I'm not sure I can vote for Hillary if Bernie loses" stuff is complete nonsense and a total farce (even you're not that good of an actress Susan) and neither woman has spent a good part of their lives in the poor or working class or as an ethnic minority in an urban setting so they represent neither the type of people most likely to benefit from Bernie's policies today nor the very people his campaign would most likely benefit from getting votes from. Look it up, they both grew up in New England suburbs, middle class, went to Northeast colleges, and got their big breaks in entertainment in their early-to-mid 20s so they pretty much are in the total sweet spot to be Bernie supporters, Millennial or not.
So unless these celebrities have some kind of deep connections to voters in Wisconsin, Pennsylvania and Maryland that I don't know about, I don't think it'll matter. If Bernie loses all those states (and New York where Hilary was a Senator), he should do the right thing and drop out before California. These celebs and the wealthy folks in California like Sarah or Susan are donors Bernie should fundamentally agree need to start supporting Hillary to make sure Trump doesn't become our next President.
I really wanted Mike Bloomberg to run, but he did the math and realized supporting Hillary (unofficially but likely) was the best and more realistic way to make sure Trump doesn't become the President, and soon enough, Bernie needs to do the right thing and follow suit. Right now, he's still putting his campaign before America despite the millions of voters and dozens of delegates he's behind in relation to Hillary so far, and that's ok because he's got the funds to keep going even if not the diverse voter base to show, but after the upcoming East Coast primaries, I hope he puts America first.