I think the very fact that she never became the numero uno is the main reason why she managed to survive and sustain her career for so long.
When you are number 1 -- which inclues consistent hit films, critical appreciation, popularity etc -- people get bored of you in a while. When the media overexposes anyone all the time, the audience gets bored of them after a while, and they disappear. Case in point: Rani was like the number one 3 years in a row (2004-2006), and very close to the number one in the other years. Audience eventually got bored of her and then forgot her in an instant. Vidya also had a similar phase from 2009-2012 where everyone was going gaga over her -- and then suddenly ppl lost interest and she fizzled out (for the most part).
Kareena on the other hand never had this numero uno crown so people never got thoroughly sick of her. When you peak high, you also fall very low (like Rani/Preity). On the other hand, when you have a decent regime like Kareena, you remain there.
As for why she never was number one: she never could give consistent hits nor was she consistent in her performance. Like in the early phase, she was really bad in her acting (Poo phase), then she got better but mostly did this arthouse flicks, then had an inconsistent graph (like sometimes giving JWM, 3I, but followed by number of flops). Hence she never was number 1 -- but that actually worked in her favor.
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