For all we know this was Bruce's choice (maybe a well thought out plan). And everyone around him went along with it. When you're with someone constantly, it can be hard to see how much of a decline they've had.
Likewise, people who aren't around them constantly will see it in the absolute worst way. It's one of the big problems for people with disabilities; people underestimate how capable they actually are.
Look at the quote in the article, where Bruce says "I know why you're here, I know why you're here, but I don't know why I'm here." I can imagine Bruce Willis saying that while being in perfect health. I can imagine many actors saying the very same thing. But it's easy to read that in multiple ways.
And these days, with multi-generational households largely a thing of the past, and elderly people shoved in nursing homes and forgotten, it's hard for the average person to comprehend what dealing with a decline in mental (and physical) health is like. It's scary that poor caregiving (by family) is being criminalized in many ways, while poor caregiving by nursing homes is largely overlooked.