When you wake up, you will almost certainly have won (a trillionth of the prize). The subsequent destruction of winners (sort of—see below) reduces your probability of being the surviving winner back to one in a billion.
Merging N people into 1 is the destruction of N-1 people—the process may be symmetrical but each of the N can only contribute 1/N of themself to the outcome.
The idea of being (N-1)/N th killed may seem a little odd at first, but less so if you compare it to the case where half of one person’s brain is merged with half of a different person’s (and the leftovers discarded).
EDIT: Note that when the trillion were told they won, they were actually being lied to—they had won a trillionth part of the prize, one way or another.
If you mean that a quantitative merge on a digital computer is generally impossible, you may be right. But the example I gave suggests that merging is death in the general case, and is presumably so even for identical merges, which can be done on a computer.