This means you have to have enough stacks already/be fed enough or active enough in order to chase down and get them solely off of champions. However, I simply feel like having it on a 6 minute timer waiting for baron to respawn and forcing your team to attempt baron every single time it's up is a little overkill. Alternating between Baron and Dragon even would help.
Edit: Alongside the fact that you're never told by the champions kit or any tooltip that after a certain point you will be forced to get stacks from only baron or champions. It only says "Enemy jungle camps" Seems like pre-6 you should only worry about hording as many stacks as possible because they're few and far between afterwords.
Edit x2: Did just realize you meant after 6 stacks instead of level 6. So that makes it a balancing tool then, was never notified that baron/Rift Herald would be the sole target of stacks. Still bummed that it's planned that way and not meant to alternate at the very least. Really caps one of the 5 endless stacking champions potential in a way not many of the others are. Really wish this was a bug.
[zombie-nunu-tears]