You are right. I've noticed a change in Topsiders too. I bought my first pair as a college freshman in 1974 and have worn them ever since. I'm fortunate to live a fairly casual lifestyle and have the opportunity to wear Topsiders almost every day, so I always have several pair ranging in condition from newer ones that "look nice" to my older "grungy work" shoes.
Topsiders used to last forever but I've experienced the same slip & slide effect when walking downhill across grass or pine straw in my older ones. You can bust your tail real quick. The rubber soles on the older, more grungy ones look like they age differently and get hard and lose their pliability. I always thought is was a chemical reaction to something I stepped in like gasoline, deck stain, lawn chemicals, etc. but maybe not.
I have worn the soles clean off many a pair of Topsiders but for the last few years I've noticed that is not the case anymore. The soles get so slippery I throw them away before they wear out.
But I still love the old classic brown Topsiders.