I worry that such a tax would create a self-reinforcing monopolistic effect by making it harder for smaller companies to do it, thus enriching those that can afford to do it. Even if there's a threshold under which it's not taxed, it still benefits big corporations.

That's the benefit of just such a "Microsoft model": one throat to choke, as a manager once told me. A tightly regulated and taxed ad monopoly system would be a lot tamer, at least until it captures the regulators.