I don't see Sanders with any risk of dropping below 15%. If anything all polls don't consider the unlikely voter which Sanders is tapping to & under-estimate young voter turnout based on previous turnout levels. That coupled up with the energy of his supporters & huge volunteer & donor base means he will very likely exceed the polls. Just look @ the number of donors of Bernie. It is virtually impossible to fall below 15%.
Are these your thoughts or are they from another site? The 15% threshold is by precinct so in a tight 4 way race with so many options he could very well slip below 15% in many communities.
Of course a lot could change in the next few months, at this point I would put him 3rd at best regardless of his percentage.
Have there been any additional polls confirming that Sanders is virtually getting 0% of Clinton caucus supporters from 2016? That's also horrible for him.