I think the loss of viewers started from the bandhua track, earlier Udaan was always ranked from 6th to 8th, the first time they were out of the top 10 was during that track. Even after the end of the track, Udaan never recovered the same trp numbers or stability.
People want happy tracks, the only time Udaan was slot leader after January was the week after confession when viewers expected romance and got baby drama.
The second wave of loss was in the baby track, Udaan never regained the viewers they lost during IPL because the track went on forever and was very negative and controversial with misleading promos. For example, there was a surge in viewers for the divorce week when the promo showed Chakor seeing Ragini's truth and it said will Chakor save her marriage? instead the divorce happened and they introduced Ajay, numbers didn't recover until the end of the track.
There is still hope, there was an increase of 0.3 trp points for the reconciliation, but CVs went for shocking twists and negative Friday precaps which lost them the viewers.
There is another general problem unrelated to Udaan, viewers generally have become less loyal and will switch from one show to another depending on track. Only the top 5 are the same from one week to another (this changed this week), the bottom 5 keep changing. Udaan was bound to suffer. Previously shows had to be above 2.3 to be in top 10, now 2 is usually enough to get you there.
In summary, number of loyal viewers have decreased and will never be back to the same level, but that isn't fatal to the show if CVs treat it wisely and start having a stable happy track before introducing a new twist.