First off, this exact opposite candidate of Bernie ran in 2016 and we saw barely any gain in down ballot races. Second of all, acting as if there's this enormous bloc of swing voters that are squishy and voted GOP but switched to Democrats in 2018 and suddenly Bernie is going to scare them away is, at best, bullshit. There will undoubtedly be people who may be turned off, but there is no scenario where the House is going to flip back to GOP or even come close with an energized voting base that Bernie has. This idea of this magnificent swing voters is mostly bullshit anyways, people identify with a party and sometimes they do or they don't come out. Trump being on the ticket is going to motivate a LOT of Democrats to come out regardless of what they think about Bernie, the ones who are suggesting that this isn't going to happen have said that Bernie had no chance in hell at winning the nomination and yet he is the one that keeps rising in the polls.
https://www.politico.com/news/magazine/2020/02/06/rachel-bitecofer-profile-election-forecasting-new-theory-108944