That's why such change needs to be enforced simultaneously and externally.
As an example, I recall reading that tobacco companies were actually very happy about regulations limiting the marketing of tobacco products - by themselves, those regulations didn't change anything about their market share (the market was already saturated), but everyone got to stop spending so much on advertising.
I don't think Marlboro cared about newcomers, a big company can usually buy out the small one if it gets dangerous. Consumers usually don't even notice.
Unrestricted advertising is basically Red Queen's race; capping it levels the playing field, so it's better for newcomers as well.
As an example, I recall reading that tobacco companies were actually very happy about regulations limiting the marketing of tobacco products - by themselves, those regulations didn't change anything about their market share (the market was already saturated), but everyone got to stop spending so much on advertising.