In general I agree, but once a business becomes powerful enough it may be able to reduce competition in what otherwise might be competitive markets.
Lets say at some point drivers of GM vehicles buy 60% of the gas sold. Gas stations must agree to use proprietary connectors to service GM cars. Because of GM's market share, the majority of gas stations decide to use GM-only connectors, which then sells more GM cars...
Such a cycle is great for GM but probably not good for consumers if you think competitive markets are pro-consumer. And it affects people who don't even buy GM cars, because it influences what 3rd party business are viable. I think that's a key difference between companies with enormous market share engaging in anti-competitive behavior and companies with minority market share (like BMW) doing it.
As another example, I think it would have been unfortunate if home internet service providers (of which many people have a choice of only one or two, and it's a big hassle to change) became a bottleneck for other types of services such as online streaming (perhaps by charging exorbitant bandwidth costs to competitors). Luckily I think that ship has mostly sailed, and even in the absence of regulation abuses are probably somewhat kept in check by the very real possibility that regulation will happen if things go too far.
I think there's an argument to be made that it would be bad for a duopoly in the mobile OS market (Apple/Google) to lead to a reduced level of competition in app stores, payment processing, and possibly computer software in general (because of the approval process). And even though Apple may not have an absolute majority on market share, the percent of software revenue is probably more important and Apple's share of that is probably quite high.
There's probably reasonable ways to address this too. Maybe by allowing other app stores (that follow the same sandboxing / security rules), which seems to work out fine on platforms like Windows with options like Steam. And maybe they could let Apple continue to require support for a common payment method, but not let them prohibit other payment methods or require that those other payment methods are the same price (when the fees on the other payment methods may be much less than 30%).