Mergers seem like the smaller concern for innovation. The big concern from the point of view of a startup is "what if BigCorp just imitates your product and puts you out of business?"

Historically, this situation has been managed through intellectual property laws, not antitrust. But patents as applied to software are a kludge, both tying up tech that should be free (particularly when that technology is a file format) and failing to protect innovations that should reward their developers. And copyright just doesn't cut it.