Alfie is more the exception than the norm. If you want to look at a more comparable career trajectory, look at Corey Perry for example. And that is the case for many players who have slowed down because of injuries... Look no further than the guys before him on your list...
Heatley hasn't been effective since 2010-11 and a massive anchor contract for SJ then the Wild (like uch more than Ryan, if you realize that the cap went up a lot since)
Spezza declined vastly 3 years ago, massive anchor the last 2 years of his 4 years contract
Eric Staal had a big decline his last 3 years with the Canes but had a surprising resurgence with the Wild. Not sure about his injury history though
Malkin is a generational player, would have went 1st OA in most drafts but that draft somehow had 2 of the greatest in history with Ovie.
Bobby Ryan was a heck of a player when he was young, healthier and the game was slower in general. He's now 32 and has a relatively long injury history, notably his hands that made him lose his sniping ability.
He still posted ~0.7 PPG in his first 3 years in Ottawa, which is/was 1st line production. In his 4th year he struggled with a crapload of injuries but totally redeemed himself with monster playoffs, which allowed the Sens to pass the Bruins. Without him, we lose in the 1st round and we wouldn't have had that incredible run before our ****ty rebuild.
He's now 32 y/o and he has never been a speed merchant to begin with so it reduces his effectiveness overall but he can still be effective and create offense because of obvious high end skill and hockey IQ. He's like an older slighter version of Mark Stone. I could see Mark Stone become similar in his 30s but hopefully he doesn't run into injury trouble like Bobby did. Mark seems to be strong as hell so I am hopeful.
That being said, even though he declined in 2016-17, Guy Boucher was the new coach and it hurt several players offensive production. Considering Ryan's usage since (TOI/GP, QoT, most minutes from the "3rd line"), I consider his offensive production to be quite valuable. 0.53 PPG in 2017-18 and 0.54 PPG in 2018-19
This year, he has 0.44 PPG in 9 games but posting better numbers than that on a team without any 1st liner (Tkachuk not there yet, so Chabot being basically the only "starting 5" caliber player) will be quite the challenge. He would even have another point if the goal against Toronto was not called back (so that'd be 0.56 PPG). Anyway, season is still very young
He's not going to be among the best 2nd OA picks ever, but a solid career nonetheless. Better than JVR, Staal, Murray, Reinhart too probably. That 2005 draft really wasn't that great outside of Crosby, some D-men and some goalies. Kopitar was slated slower (right at our 9th OA pick!) and Stastny was picked in the 2nd round, same with Neal. Oshie was a late 1st.
That being said, for the crap that he took, he still outscores Silfverberg 7 years later lol (262 pts in 440 GP vs 241 pts in 455 GP), despite less TOI/GP and in general less top-6 TOI/GP. Noesen was waived by the Ducks and Ritchie is a marginal 3rd liner (looks more like a less effective prime Zack Smith than anything else) so this trade was really the "disaster" it was said ad nausea by the usuals.
https://thehockeywriters.com/nick-ritchie-anaheim-ducks-move-on/