In the early plans the harpoon was launched at a moving target. But in addition to the complexity and the risk associated with that, this prevented having a close-up look at the target after the operation. A harpoon for debris removal is only useful if it does not create too many debris and you cannot really assess how many were generated if the harpoon and it's target are floating away.

I concur with you in the drag sail. So many LEO projects are planning on using propulsion for deorbiting, which requires having a fairly working spacecraft. Drag sails or other deployables do not require a lot of power, are completely passive once deployed, and make the object even more trackable.