The mere fact that competitors oppose a deal is very strong evidence that the deal is in fact pro-competitive. If a deal was going to lead to reduced competition, competitors should be jumping for joy over it: they would benefit just as much from reduced competition, at the expense of consumers. If they're against it, it must be because they think that the combined company will in fact be more competitive, forcing them to cut prices or do other pro-consumer things to keep up.

If Toyota was planning to buy Ford, and the rest of the auto industry was talking about what a great thing that was for consumers, that would be something for consumers to worry about.