Say, old Buttercup had been a reliable race stallion through the years. He had his uses, he was always there when you needed him, and he probably made you a good amount of money at the track. But with success comes pressure, and the good times can't last forever.
Mario's doing pretty well. Mario's successful, and has been for many years.
The problem isn't sequels so much, it's the annual franchises. COD every year, FIFA, Battlefield, Assassins Creed etc. It sort of started with FIFA and Pro Evo and now every publisher seems to need to have a yearly franchise, smacks of greed. But who can blame them when people still buy them, hell some people only ever buy COD n FIFA anyway. Sequels are great sometimes, GTA for example. Sometimes less is more as it creates anticipation and also gives more time for developers to create the best experiences. Now roll on Dark Souls 2.