If we look at the drop in TRPs, the show started declining in TRPs when they shifted the focus completely to the Garg family drama with marriage ceremony going on for 3 weeks or so, seema's mausi arrival, seema-sangeeta-mausi scheming, and that 3 week long Mayan honeymoon.
You can show the romance but for how long. You also need a strong storyline.
The show was at its peak in terms of TRPs when they had interesting college tracks and there was some balance in the storyline.
The decline in TRPs is not sudden and abrupt. SSLK is the same show that was in the top 5 for several weeks and was even number 1 at one point (I think only a week or so).
No one wants to see only saas-bahu drama and endless scheming.