Since Trump has about 40% now, do you really think all the others supporters and the undecided are going to rally around Cruz? He would need something like 90% of them to pull ahead. Believe what you want, but it’s not going to happen.
1. Cruz has to prove himself to be a viable alternative, and I think he is. and
2. That 40% isn’t all hard supporters. I’m sure a significant portion of that is soft support that can be moved (doesn’t mean it will)
3. As the link to Erickson’s post points out Cruz’s supporters may be more motivated than Trumps in Iowa, if that happens and Cruz wins that caucus perceptions will shift and what I point out in #2 will be a bigger factor than it currently is.
This is all just my humble opinion, of course. :-)