There would always be companies donating to charities and advertising - just only the ones below the threshold. And the variety and number would be a lot more vibrant

Word-of-mouth would actually be what the mega corps would aim for in this hypothetical system. They would be forced to treat customers super nice and actually lose money. Consumers would win and competition would improve. I propose the increased number of competitors would actually increase the donations made to charities. It just wouldn't all come from Coca Cola Corp and instead from hundreds of others