The people saying Bernie can never beat Trump are the same ones who were saying Trump can never beat Hillary 4 yrs ago.
Trump had a message that appealed to a lot of people in the center. That’s how he won most of the rust belt.
Sanders has a message that only appeals to the far left. Seriously, only 25 to 30 of the entire country thinks Bernie’s nonsense is a viable plan for the nation.
While you are 100% correct about anything being possible, I don’t think Bernie has a realistic chance at winning the general election. Any of his gains he might get from young voters 18-29 will be wiped out by voters over 30.