There's no such thing as a viable Biden campaign. To put things into perspective, a few weeks ago Biden was supposedly to win SC by 40 points. Look at him now - he will do ok to just win.
That's simply not true. Not a single poll in this entire election cycle has shown Biden 40 points ahead. Through mid-2019 (e.g. June, July) there were a number of polls showing him up by between 20 and 30 points. From November to December he was ahead on polling average by 20 points, narrowing to 15 points in January and 5 points in more recent weeks. But the polling average lead has now blown out to 15 points again. If he wins by that sort of margin, it will show that he has come back from a much tighter polling position, despite his apparent lack of momentum. If he wins by 20 points of more, he will likely become the national vote and delegate leader.
The 538 forecast currently has him winning Texas, North Carolina, Virginia, Tennessee, Alabama, Oklahoma, and Arkansas on Super Tuesday. By contrast, Sanders is ahead in 6 contests. The 'narrative' coming out of Super Tuesday will likely focus on how many states a candidate wins.
That forecast takes into account the assumed win by Biden in South Carolina. But if he over-performs the expected percentage, those chances will go up.
Overall, his current chances of securing a plurality of delegates are 16%, compared with 32% for Sanders.
Which is not to say that this will happen; but rather than it is simply false to say his campaign is not viable. He is the chief opponent to Sanders at the moment, and the gap isn't as large as the 3 results so far would suggest.