Their monopoly is preferable to many alternatives, but we need to remember:
It is a monopoly.
Their 30% cut is egregious.
The latter should be a simple argument, but I've had some bewildering interactions where people cannot grasp 'number too big' unless I name a precise lower value and stake my life on it. Nah. The simple fact is, it's the same gross cut that console manufacturers take. Those companies design and manufacture the underlying hardware, and ruthlessly maintain absolute control over what games are permitted on the platform. So it's eyebrow-raising that a mere independent repository can also scrape off a third of the retail price for games they didn't make.
The former is a simple argument, but only when people can be honest about words. Every denial boils down to 'but I like Valve!' no matter how thoroughly I establish, so do I. They're doing fine. They're still a monopoly. There's one store you kinda have to be on because everybody goes there... often exclusively. They have a super-duper-majority market share. Their competitors exist but do not matter. Epic has one game that makes a billion dollars a year selling hats, and even they can't move the needle.
It's fine because they're not evil. They have not abused their monopoly position for anticompetitive practices, and it's hard to argue they're anti-consumer. But the precondition for abusing a monopoly is being a monopoly.