Retail is a prime example of consolidation -- think of Walmart,
Sales by the 20 largest food retailers totaled $515.3 billion in 2016, accounting for 66.6 percent of U.S. grocery store sales, up from 42.2 percent in 1996. Amazon acquired Whole Foods in the summer of 2017. (Source)
Fast forward to 2020, "Amazon and Walmart are like two elephants wrestling, and all the other retailers in the U.S. are the grass" (Source).
Telecoms is concentrated, though it's become less concentrated in recent decades.
Telecom is another classic example of consolidation.
A long antitrust case completed in 1984 led to the old AT&T being broken into seven regional Bell operating companies and the much smaller new AT&T.
Less than 30 years later, in 2009 the Department of Justice started to look into whether AT&T and Verizon were abusing the market power they had amassed in recent years.
Now... you don't have to take my word for it. Google for "consolidation in _____ industry" and you are going to see the phenomenon being repeated everywhere.
Economists say free trade is good because of "comparative advantage". But what is comparative advantage? Why is it good?