That's a possibility, but that still wouldn't be more intrusive than the current sponsored tweets. If anything, if this model is successful, they might even reduce the amount of sponsored stuff appearing in your timeline.
"Number of ads to show" is something that can be optimized. Too much ads have a negative effect on your entire platform, so you need to strike the balance. 'As much as possible, but not too much'. If they can improve perception of their platform and improve the view-to-click ratios of their buy buttons by showing less sponsored content and rejecting customers, they will. A company always tries to find the economic optimum.