TOFO, I was lucky to fly in new aircraft throughout my career with the exception of 24 year old Shacks, even the Anson was only 20 and the Lanc 25.
I last flew in the Nimrod when it was a bare 10 years old. I was invited on board one at RIAT when it was then 35 years old. I was amazed and in a way distressed at how shabby it was.
Regardless of the magic kit being upgraded, if modern aircraft are to serve 35 years and more then the whole plane needs to be refurbished.