[HumanReadable]

Why not? In abstract the argument is:

"I want this product to be better"

The reason we don't want companies to gain market power (the feared result of mega-conglomeration) is because it allows companies to charge more for the same or a worse product.

If this person believes that Blizzard being acquired by microsoft will result in a better product, then they expect the net benefits to outweigh the negatives of Microsoft's increased market power over the video games market.

Acquisitions that produce a consumer-surplus should be most welcome!