Jones wasn't slowing down. I feel after watching MMA for a long time you should realize early 30's ain't old in this sport. LHW's like all MMA fighters are bigger better athletes then they were when Jones became champ(Rua, Rampage,Machida 6 foot 1, Belfort 6 foot 0, Evans, DC 5 foot 11). Jones used to be much bigger than all his opponents those days are gone and have been for a while. Part of Izzy's lesser dominance today is cause he's the same height as Jones but at 185.
Anyhow there's like 10 LHW's who'd beat Jan that doesn't mean much. He's champ cause everything broke the right way for him mostly cause Dana put Rockhold and Jacare in line for title shots they didn't deserve based off name recognition. The result was Jan getting credit he didn't deserve for beating them.
a)Bisping stole Rockholds soul and doesn't know how to put his hands over his head.
b)Barely beat Jacare. Beating Jacare has become a MW rite of passage.
c)Corey Anderson dominated their first fight and Jan flukily flash KO'd him in rematch
d)Reyes took Jan lightly cause he thought he was hot shit for coming so close to beating Jones(which is understandable I think most of us would but he paid the price).
Jan has shown no history of KO power outside of the 2 fights listed above and was winning most of his fights by decision. He was also a gatekeeper before this run always losing to the best guys he faced most recently to Thiago Santos. While GSP might have targeted a weaker MW champ, Bisping had KO'd Rockhold with 1 eye and in the previous era had been an elite MW. The fact Bisping hadn't been champ or at least gotten a shot earlier was as much bad luck as beating Rockhold was good luck. Jan was never an elite LHW including in the late 2010s when the division was at it's absolute weakest.
With Izzy it's hard to rate him cause MW fell apart when he was coming on the scene. Almost getting KO'd by Gastelum a fighter with horrid cardio is not a good sign and Izzy hasn't shown much in his last few fights and at MW even if he racks up a bunch of defenses he can't prove anything cause there's no one else really there. Beating Jan to be champ champ which he'll most likely do won't prove anything cause he's a lower level LHW who lucked his way into being champ. It's exactly like what AS did with Forrest except Izzy gets a belt.
Jones on the other hand was the opposite beating the best division in the history of the sport where you had 4 psuedo champs then TRTor. IMO he's been the GOAT ever since. He's only fought once a year since but everything else is just a bonus with Izzy it's the opposite cause his early defenses came against arguably the worst division in MMA history. Whitakers really all that's left of that era(Romero's complicated cause the records swear Izzy fought Romero but I swear they never fought). Costa might have been unbeaten but before the Yoel fight he was fighting guys like Johnny Hendricks and Uriah Hall.
Point is I don't understand your faith in Izzy being something special nor why being favored to beat Jan is an accomplishment. Bellators whole top 5 would be favored to beat Jan.