Kartik was introduced within the original cast that is when we had all maheshwari & singhanias - so naira and kartik getting into lead's shoes was gradual as akshara naitik were still the lead couple until akshara died.
Also, Moshin was not such a house-hold name until Kaira flourished so audience had to be made fall in love with his character as it was obvious he will be future lead. After Moshin & Shivangi's success as kaira CVs needed to make sure they have audience so they got harshad who already has a big fanbase and needs no time to get audience hooked to the lead pair.
If abhimanyu was introduced as go-to doctor for one of the goenka's member or not promoted as best doctor in Udaipur, abhi's intensity after falling in love would have been more interesting to watch.
Its best if we treat YRKKH 3 as new show where we just happen to know the backstory of one side of family (Goenka's).