My parents cat went similarly. When they go downhill, it can happen very fast. Possibly it's better that way than long and drawn out.
I've known a few cats who were well into their third decade, and as you say, they had an easy life. Mostly they'd wander between their bed, their toilet (aka the flower bed), and lunch. On the other hand, that's not vastly different from a young healthy cat!
She does look like a smashing cat in that photo.
