Actually, it's not only that Netflix is popular, but also that Netflix content is heavy... so fewer user are generating more IP traffic comparing to another website as popular as Netflix

Anyway, in the end, the ISP may just "limit Netflix bandwidth". Then he will lose some customers (changing ISP)... but Netflix will lose some customers too (leaving Netflix because of bad quality compared to another content provider). This would be a lose/lose game

Or both may agree to share a part of the burden of "heavy contents", most for ISP et a bit for Netflix