[plausibility]

Look no further for evidence than New Zealand. There are two major grocery store chains (Foodstuffs, who own New World, Four Square, and Pak n Save) and Woolworths Group -- obviously we have the smaller Asian marts and produce stores too, but most people only have one of the big stores nearby to their towns.

There are two major building materials suppliers (Carters and Fletchers). There's one manufacturer of drywall (Gib) that is easier to get council plan approval for than any other cheaper manufacturers of drywall because they provide some material strength documents that saves the councils some engineering review time and effort.

We technically have 4 major banks, but 3 of them are just offshoots of big Australian banks and siphon the insane profits offshore.

The government keeps making investigation commissions into breaking these up, but doesn't do anything. The companies just point fingers back and forth at each other blaming "the competition" for price gouging. Meanwhile the recommendation from the politicians is we cut back on avocado toast, lattes, and our Netflix subscription.

[usr1106]

Finland is nearly as bad. 20 years ago there were only 3 food chains. All domestic and playing the rules "no price competition". So food prices were about 30% higher than in Germany for example (these a very different countries so lack of competition is only one reason). Then Lidl (discounter of German origin) entered the market. The first years the incumbents fought it with unfair practices, but in the end it led to more price competition with everybody having to offer cheaper choices. 2 of the incumbents have since merged (with some regulatory limitations) so we are back to 3 players, 2 playing "according to the oligopoly book" and one doing things different, at least offering some choice.

Banks are not much better. There are a couple of small players additionally to the 3 big ones, but competition is limited to very few products. If you are interested in something else, choices are very poor.