Granted I have a ball over 20 years old I still use from time to time, but how often do you have a ball as your favorite go to ball more then 2 years? The ball will usually still be useable longer than 2 years. But I bet most of you probably buy a new ball on average every 2 years anyway. So while a four year warnty is nice Hammer was 3 before Brunswick took them over. 2 years is probably sufficient. Balls that have been on the market more then two years in a lot of cases are hard to find never been grilled anyway if the bell was released more then 2 years ago anyway. So you won't be getting the same type of ball anyway.
I agree 4 year warranty is nice. But if the ball is more then 2 years old already you probably already are looking for a new go to favorite ball anyway, or if the ball surfers damage that the warranty would cover odds are its going to happen after the ball has already been retired so you would be getting something else, or the same type of ball, but used off of eBay or something like that. So probably between 90 & 90 percent of the time. No matter which ball is your go to brand. Storm, Brunswick, Motiv and all the rest a 2 year warranty on a brand new never drilled out of the box bowling ball is in most cases sufficient, or at least it should be.
I am sure market research has shown that the average league bowler even the ones with higher sport averages probably purchase a new ball on average at least once every 2 years. Some might try to go 3. But most its probably at least one new main ball every 2 years. Some hardcore bowling ball junkies might by 2 if they bowl on a lot of different lane conditions.