Bernie Sanders probably has a support ceiling, but there are still several ways he could win the nomination

Every candidate has a support ceiling. Beyond that, to reply to some of his points:

3. "Sanders has gotten relatively little bounce in the polls from his outright win in New Hampshire and popular-vote win in Iowa."

I'd say some of this is due to the generally lackluster media coverage of his win, which generally portrayed Pete and Amy as the biggest winners simply because they did better than expected, and generally painted his NH win as a loss because he didn't win by as much as he did in 2016.

4. "Even in fairly liberal states like New Hampshire, voters seem to prefer a more moderate candidate in the abstract. [snip] And 50 percent of voters said Sanders’s positions were too liberal."

Findings like this seem to be rooted in people's fear of labels. Entrance polls showed nearly 6/10 caucusers in Iowa and nearly 7/10 NH voters said they supported a single payer healthcare system.

5. "Turnout in Iowa was 176,000 people, about what it was in 2016 but well below 2008 levels and less than what most observers expected."

I'd attribute at least some small part of this to the different timing of each of these. In 2008, the caucus was held January 3, whereas this year it was February 3. In 2008, this was likely over winter break at many schools, bringing students back home to the state.