Well they already said they will bring SP chains for grind parties.

That being said, I love your idea of temporary buffs upon reaching a certain # of chains. Maybe instead of loosing the buff when the chain is lost (since it should be hard to get past chain #5) the buff could last like 5min. So a grind party would strive to get to chain #5 to be able to keep going.

Or your idea is good too, the more chains you do you get more buffs that goes away once the chain breaks so that essentially it would be a challenge to maintain those buffs and keep the chain going.