I don't see how serving scam ads would impact google, they still get paid to do it

Short term, sure. But they are not a monopoly.

Maybe not in legal terms. However a 90% market share in a market where the majority of consumers never evaluates switching to a competitor, gives you so much leeway in execution, that there isn't much difference to a monopoly.

> never evaluates switching to a competitor

Again: “short term, sure”

Unless you have a convincing case for why “never” evaluating switching will last forever.