Datsyuk hasn't declined much. I'm a Red Wings fan so I watch most of the games. He is still easily the best defensive forward and provides much of the creativity our aging set of forwards as well.
I think there are Datsyuk, Malkin, Crosby who are pretty much HoF locks and Stamkos who will likely be there some day as well. The others I regard as just very good players during this period of the league. In the last generation you had Modano, Sakic, Yzerman, Forsberg and then below them guys like Kariya, Yashin, Bure, Lindros, LeClair.
Parise is a very good player but I'd still rather have Zetterberg, for example, and I don't think Zetterberg is in the top tier.
The Kings went on a great run but they were an anomaly. Hockey is the sport where one player can singlehandedly win a series (Curtis Joseph used to do this all the time for Toronto) but the fact that there is more variance in the NHL playoffs doesn't mean stuff like the Kings will happen often. If I recall correctly, the Kings were one of the best teams in the entire league after the trade deadline.
I did say Datsyuk was still a hell of a player, just that there's no doubt in my mind he was better than he is currently, and there's nothing wrong with that really, it's bound to happen when you get older.
Kariya definitely had more than enough skill to be as big as Forsberg & co. but was unlucky with injuries and other setbacks, I should know cause I'm "a follower". He was my favorite player when I was little and I used to watch games that my dad recorded for me since staying up till 2 am (when the games usualy start) wasn't an option back then.
Just because it happened doesn't mean it will again, I agree, but it's certainly the first sport I'd point to if we were talking about the outcome being unpredictable.
The Kings were one of the best teams towards the end of the season, they did get better at half point when Sutter was appointed as coach, but he also needed time in order to make them play up to their potential.
And another thing I'd like to add, since you mentioned Datsyuk easily being the best defensive forward, I don't think that's the case honestly.
I mean he COULD still be the best, but it's certainly not a statement that no one would argue.
For example I think Kopitar is one of the best (if not slowly becoming the best) two-way centers in the game. And I bet a lot of other people could make a case for some players that are right up there as well.
You being a Wings fan makes you somewhat biased up to a point, even if you're generaly objective you can't help but feel that little bit better about Wings players, which is one thing I can't exactly plead innocence in either.
I love Kopitar ( duh, Slovenian
), and subsequently love the Kings, and even though I think of myself as being a really fair objective fan in terms of addressing players' qualities, I think everyone tends to see the team/player they cheer for in a bit of a better light than others.