Until Super Tuesday, Sanders was viable. Those who have no intention to vote for Trump had to choose which Dem best represents their view.
That's speculation not evidence. And one could equally well speculate that Dem voters thought (rightly or wrongly) that Trump with his high negatives was so eminently beatable in November that getting him as the Dems' opponent was the highest priority.
Your argument makes no sense. Look at Trump’s rallies. A lot of enthusiastic young voters are there.