The figures I've seen are 40% with the combination of CPR and defilbrator if the defilbrator is applied within 5 minutes. (Swedish national "CPR register", several thousand cases). If you don't do CPR but still apply the defilbrator within five minutes the survival rate is 20%. The risk of permanent nevrological damage also increases massively. If the defilbrator is applied in the 6-10 minutes window the survival rate is 30% with CPR and 10% without.

Add to this that the quality of CPR is likely to vary considerably among these cases, but it's close to impossible to correct for that. There is no way of knowing how many of the fatalities that could have lived if they had received higher quality CPR.

Yes, more than half of the people with a stopped heart will die. But immediate CPR does save lives - if you can apply defilbrator within minuttes. And advanced hospital treatment within hours is absolutely essential for the quality of life for those you're able to resurrect.

I have no information about how the survival rate is distributed across fitness and age, but I wouldn't be surprised if the chances are much better for a healty, not too old person that receives immediate CPR followed up by quick defilbrator and a trip to the hospital. Heart failure isn't something that exclusively happen to people in very poor health. Even athletes die from hearth related problems.