Treat customers right and you get rewarded. They are the majority market shareholder because they have earned it, not through deceptive business practices but through being a great company.
If they were a monopoly they wouldn't allow other game catalogs on their systems, yet I have GOG and Epic on my Steam Deck. In fact, there isn't even a requirement for me to have Steam on my Steam Deck. Just because a company is the market leader doesn't mean they got there through unethical means.