They tried the same on Trump in 2016, With candidates dropping out and even Fox News laughed at Trump before he won the primary, but Trump prevailed. If Bernie really had that much support then nothing was going to stop him but thats not what happened. That's the bar im setting on that situation, not denying forces were against him, but a truly strong candidate would have won anyways.
Considering he had no support from his own party I would say he was a pretty strong candidate. And with the amount of money they spent crushing him I would say they considered him a valid threat.