Rav Belsky specifically addressed the “If you weren’t going to buy it otherwise” taina and said it doesnt work. Based on what I understand from what he said the reason is since you obviously wanted the song otherwise you wouldn’t have downloaded it, that means that there is an amount that you would pay for the song ergo its still hasagas gvul. A bit of extrapolation on my part but he clearly disagrees with the concept of the taina.
Rav Simcha Bunim Cohen has also specifically addressed this point and concurs with Rav Belsky. He didn’t have time to explain his reasoning though.