You want to promote your post for visibility? 3.5 days is more than enough. Afterwards you'd primarily do it for monetary gain.

And regarding the self-voting: it's his stake and downvoting isn't giving any rewards.

Minnows self voting isn't giving much of any rewards either. When they buy votes, they are doing so with their stake. The vote sellers, likewise, are also using their stake. I wouldn't be surprised if grumpy cat was a vote seller himself, or a group of them. That would explain his incentive.

The fact of the matter is, grumpy often upvotes himself on the last day. If you're saying that's ok, then minnows doing the same is also ok. In fact, minnows buying votes are also taking on risk which he is not.

Additionally, being a capitulating service, you have a vested interest in supporting his activities. By strongarming customers, he is decreasing your competition.