It's not a monopoly when a company you don't like buys a chain that you did like and ruins the experience. It sucks, but it's not a problem for antitrust unless the acquisition creates unfair competition in the target industry.
I didn't pick the grocery industry, OP did, I was just working with what they were discussing.
Anyway, the question isn't whether grocery stores have a consolidation problem (they do), it's whether Amazon's purchase of Whole Foods made that problem better, worse, or unchanged.
Looking at the market, it's hard to argue that Amazon buying Whole Foods made the consolidation worse, because Amazon wasn't a grocery store player until they bought it. And, in fact, it's likely that Amazon's purchase could slow the consolidation because they are better able to keep Whole Foods independent of the existing conglomerates than Whole Foods could while operating alone. If an acquisition was inevitable, better Amazon than Kroger.
It's not a monopoly when a company you don't like buys a chain that you did like and ruins the experience. It sucks, but it's not a problem for antitrust unless the acquisition creates unfair competition in the target industry.