I just helped my neighbor take down an old DirecTV dish after a Slimline install by DirecTV. The tech told him that since it was in a different location and he was running all new cable it didn't have to be removed for the new install so he wasn't going to do it. Of course this same tech put up a Slimline without monopoles and it supposedly passed QC.
I suspect it was an HSP tech that was more worried about how he was going to finish the four 1-5pm installs they routed him than about the old dish still being on the side of the house.
Honestly, he wasn't out of line. I wouldn't have done it either. Every trip up the ladder has a chance to end in disaster, and I am not swinging my ass in the wind for free. So, unless it was directly in my way, it stays.
Monopoles are always required.