You are wrong.. Casting is very important to improve the slot..
Before to Namma Lacchi, Bettada Hoo used to telecast, which had a constant ~2.3 TVR. But later once Namma Lacchi started, because of popular actors and content, it gained viewers every week and even crossed 6+ TVR by giving tough competition to Gattimela. Zee on the other hand started boosting 7.30pm PM, mahasanchike etc to retain 8pm slot viewers. Due to this, Namma Lacchi started giving many twist and turns in the story and reached dead end of the story in a shortcut. Few other reasons are like,
1. BBK Vinay left the serial for BBK
2. Shanta got marriage break
3. Cost cutting episodes in the last 2 months etc when Laxmi Nivasa getting launched caused reduction in the TVR