It is hard to make the case that customer service, park conditions, ride conditions and food quality haven't gone down over the past 5-10 years. Current levels of all of the above were simply better in the past.
I wish there was some way to quantify this statement beyond just, "I remember." I've been going since 2005, and I haven't noticed any decline. Customer service? The same, excellent, top-notch service, with the very occasional grumpy-puss (we met our first grumpy castmember in 2007). Park conditions? The same, clean and well maintained. Ride conditions? Some rides are showing their age, but there were rides showing their age when we first went in 2005. What's really neat is how several of the "aging" rides have since been refurbished or re-themed or just plain yanked and had something else built in their place. Food quality? That's actually improved! When we first came in 2005, we had a heck of a time feeding our hypoglycemic son and picky daughter. It's gotten easier to find healthy alternatives every trip since then.
One of the things that keeps us returning time after time is the way Disney is constantly working on their parks. There's always something in the middle of being built or fixed. Scrims, walls, cranes... Disney's never finished. And we like it that way!
On the other hand, my mother is a professor and I grew up in campus housing. For almost forty years now (counting from when my mum started teaching), I've listened to my mother and her friends bemoan the state of today's youth. The students they are currently teaching are never as bright, well-educated, motivated or attentive to their studies as the kids they taught five or ten years ago. And it's a moving target! In five years' time, today's students will have taken on the warm glow of nostalgia and tomorrow's students will be the ones who make their teachers' despair for the future of the human race.
So, I get VERY suspicious whenever someone starts talking about how much better things were in the past. Because, in my experience, our memories lie to us.
As long as Disney continues to grow and change and innovate and expand (purchasing Marvel and Star Wars!), I'm reluctant to conclude that the company is in any kind of decline.