Most people have pointed it out already, but I want to confirm / support the idea. The punishment for CS should _definitely_ not persist after item is fully upgraded.
I'm fine with losing tribute/spoils passive after 1000g since you save 1400 which in total is 2400g from your supp item. However supps need to help waveclear in many situations mid/late game and the diminishing gold will most definitely result in less than optimal game decisions, and/or less options for gameplay. You shouldn't have to wait for any laner to go and prevent a tower from dying or crashing a wave to deny XP if you have an opportunity to do it yourself. Especially in Solo Q.
This right now feels like a bug that needs a hotfix.