Then, as I stated before, there is a problem with your theory.
A lot of voters who support Trump first have Cruz as their second choice.
For those voters to actually vote for Cruz, Trump would have to drop out which is not going to happen.
Your point of who is the second choice of whose voters is exactly the right point. Somebody with access to the micro data of surveys that include the second choice question might know the answer. But, so far, Carson’s fall has equalled Cruz’ rise.
And, let’s not forget about Trump being a second choice. In the last month or so, Trump broke through the 30 point ceiling. He’s now in the mid 30s, or the low to mid 30s if you also consider state polls. So, obviously, Trump has been the second choice of some of the candidates whose have left or who have fallen.