It's not concerning if there are actually a couple significant contenders for the top, competing in the same space in a fair market.
The problem is that when you get one dominant player, they use their dominance to change the market so that it's no longer fair, buying any competitors that approach relevance, lobbying for laws that no competitors have enough voice to combat, etc. In a fair market, consumers benefit because companies compete to provide better products/services/prices. But a single dominant player can compete in other ways, which don't benefit the consumer.
One example of this: Amazon allows third-party sellers to sell on Amazon. If you create a product, such as Iron Gym pull up bars[1], you can create an online store for it. But your store is competing with Amazon, and even though the Iron Gym pullup bar was better than the pull up bars on Amazon which required you to screw them into the wall, people would likely buy an inferior, more expensive product from Amazon simply because they didn't know your product existed. You can easily compete in the home gym equipment market, but Amazon's dominance in the online retail market forces you to sell through their platform. No big deal, you're trying to compete in the pull up bar market anyway.
So you sell your Iron Gym pull up bars on Amazon[2]. Now Amazon has all the market data on your product, and sees that it's doing well. So they release an AmazonBasics version of your product[3], price it automatically cheaper than your product because they don't have to pay a cut of their profits as an Amazon third-party seller, prioritize it higher in their search results, and wait for you to go out of business. Amazon's dominance in the online retail market lets them completely reverse dominance in the pull up bar market, without any real competition in the pull up bar market.
It might seem that this is good for consumers, because Amazon provides a cheaper product, but keep in mind that if Iron Gym goes out of business, Amazon is free to hike the price of the pull up bars up to whatever they want.
The problem is that when you get one dominant player, they use their dominance to change the market so that it's no longer fair, buying any competitors that approach relevance, lobbying for laws that no competitors have enough voice to combat, etc. In a fair market, consumers benefit because companies compete to provide better products/services/prices. But a single dominant player can compete in other ways, which don't benefit the consumer.
One example of this: Amazon allows third-party sellers to sell on Amazon. If you create a product, such as Iron Gym pull up bars[1], you can create an online store for it. But your store is competing with Amazon, and even though the Iron Gym pullup bar was better than the pull up bars on Amazon which required you to screw them into the wall, people would likely buy an inferior, more expensive product from Amazon simply because they didn't know your product existed. You can easily compete in the home gym equipment market, but Amazon's dominance in the online retail market forces you to sell through their platform. No big deal, you're trying to compete in the pull up bar market anyway.
So you sell your Iron Gym pull up bars on Amazon[2]. Now Amazon has all the market data on your product, and sees that it's doing well. So they release an AmazonBasics version of your product[3], price it automatically cheaper than your product because they don't have to pay a cut of their profits as an Amazon third-party seller, prioritize it higher in their search results, and wait for you to go out of business. Amazon's dominance in the online retail market lets them completely reverse dominance in the pull up bar market, without any real competition in the pull up bar market.
It might seem that this is good for consumers, because Amazon provides a cheaper product, but keep in mind that if Iron Gym goes out of business, Amazon is free to hike the price of the pull up bars up to whatever they want.
[1] https://www.irongym.com/products/total-upperbody-workout-bar...
[2] https://www.amazon.com/Iron-Gym-Total-Upper-Workout/dp/B001E...
[3] https://www.amazon.com/AmazonBasics-Pull-Upper-Body-Workout/...