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No, it is a problem before that. When it becomes a problem bad enough to warrant government intervention is another question.

That depends on national laws. In the UK, for example, > 25% market share makes you a monopoly.

Is that if no competitor also has 25%? Android and iOS must both be above 25% in the UK, which of those are considered a monopoly?

Don't think about the laws in terms of a single monopoly. The laws should be more accurately described as anti-trust or competition laws.

A handful of large players, each with less than 50% market share, can also distort the market and engage in anti-competitive tactics.