Change Research seems to be more Sanders friendly than the other pollsters. But still, it does appear that Sanders has regained some support, while Biden has lost some from his peak.
If such a result were to occur, only Biden and Sanders would get delegates. For the race to not be just Sanders vs Biden, someone's gotta break the 15% marker in Iowa.
You gotta keep in mind though that, unless they changed this system (and I don't think they have), this applies at each caucus location with a possibility of switching. So if this result were to happen in a bellweather Iowa caucus location, the supporters of other candidates would get to switch around.
Thing is that if only Sanders and Biden get delegates in IA, then momentum will be on their side. Voters will naturally flow towards the two from the minor candidates. At that point, a minor candidate would need to claw back momentum by doing well in the other 3 states, a rather difficult feat. If a minor candidate gets 15%(in this case it would be Buttigieg), then they can obtain some momentum and put up a strong showing.
You need that 15% to start off with to get your campaign going.
The point is that Buttigieg or Warren (for example) could easily hit the delegate threshold on these numbers due to the redistribution of people who went for nonviable candidates in each of the caucus sites.
