I'm just unsure how that is true. According to polls he's the most popular politician and he's ahead of Trump in polls in regards to an imaginary vote. His platform, whilst many will lambast, is a populist movement which the polls suggest is far more popular than the Wrestling forum would have you believe.
I'm not a massive believer in polls, but there is not a single one that would suggest Bernie would even LOSE to Trump let alone not stand a chance and his political stances are seeing even larger numbers supporting it. For example medicare for all now has a 70% approval among Americans, up from 50%, and even among Republican voters it now have a majority approval rating.
All of this is happening while Trumps pollings
are falling (Depends on the poll by how much)
I'd agree he's not as strong as before but neither is Trump, Jan 2017 his Disapproval rating was 47% and now its 53%.