That's just the thing - it's not necessary. But if you look at Bollywood through the ages - you'll notice that it is something that has always occurred. Each decade has had its Royalty, most recently would probably be Madhuri Dixit as the Queen of Bollywood - alongside her is SRK King of Bollywood (and the book By Chopra only helps the argument).
Again it's all subjective, of course any tamed mind will contest that you can never have one greatest actor - each have their strengths.... some obviously come closer to perfection than others (Amitabh Bachchan anyone?)
Ooooo, that sparks my next discussion - THE LAST LEAR