No, they are not. For example, Manchin won in a state that Trump won by 40 points (and likely is gonna win even by a larger margin this time). Democrats in blue states will eventually unite around Bernie's plans, but those in purple, red and deep red states are going to reject it.
Governing is harder than being in opposition. Just ask Republicans who bitched for 8 years about Obamacare being the root of all evil, but then couldn't repeal it even when they controlled everything.
And if you say that if Manchin is a Republican in disguise, so it doesn't matter if it is him or some Republican in the senate, you are wrong. If some Republican was in his place, Obamacare would have been repealed. At times (especially for the big issues), the margins are so thin that you need Democrat senators from the Red states, even if they are unreliable voters (especially for divisive laws). The alternative is a senate fully controlled by GOP, and good luck of implementing anything in that scenario.