I just read an article (Link: https://www.indiaforums.com/forum/karam-apnaa-apnaa/531540/yash-is-not-the-lead-of-kaa) and thought that might be the reason why Shiv was replaced and not killed.
According to the article, Hiten is the Prime lead in KAA after Iqbal's exit, so that's why they didn't kill Shiv to make the story move forward.
Think about it, all Kekta serials, the female lead is married off to some other guy she doesn't love and that's when her on-screen husband comes into play. Then a triangle love-story starts which lasts for about 1-2 years, some hardships that both have to go through before their relationship is strong enough to fight off the hero-turned-villian's love.
Then after, the generation leap comes and then... you guys know it😉.
End: And they NEVER lived happily ever after😉😆