If I was a lawyer, fighting FOR the company, I would literally use your exact same argument as for why vertical integration is a good thing.
Reducing costs, and making the market more efficient is a good thing, not a bad thing. Reducing prices is good, not bad.
However I'd argue the damning blow is that centralization makes efficient but fragile structures. Its why everyone hates a monolithic codebase.
I don't think you would want to argue that driving competitors out of business since antitrust legislation is partly designed to prevent that.
My understanding is that using the lowering of costs as an argument works when the government is concerned with consumers but not when it is concerned with competition. I think the US is the former while the EU is the latter?
Furthermore, since the parent company has removed its demand from the market, there is less money moving around. The less competitive businesses who contributed to the market forces keeping prices low end up folding or being vertically integrated into competitors. The market dies and the oligarchy strengthens immensely. Especially so because now startup competitors must have all segments of the market in order to compete.