He has a family history of dementia. Both his mom and grandfather had it. As for the liver failure, yeah, drug use was and is rampant among Hollywood and celebrities. Alcohol can be a problem, but drugs really are the issue that causes most of these organ failures and other problems. I was born in 1960 so David Cassidy, Donny Osmond and Bobby Sherman were huge parts of my young teenage life. Donny has led a clean life and is very healthy and as far as I know Bobby (he's 74 so older than David) is also doing well. David had so much baggage-divorced parents, young celebrity, exposure and availability to whatever he wanted all led to this.
As for the PF, I hope everyone realizes the only ones that actually sang were Shirley Jones and David The "band" wasn't the tv family. There were four additional performers who sang the songs.