It's the lack of activity + the level of competition he's fighting.
Facing lower quality opps like Sukhotsky, Bika, and Karpency over a 9 month period isn't a tough road for any top tier LHW to do....Andre Ward just fought Sullivan Barrera, Alexander Brand, and Sergey Kovalev over a 8 month span breh for example
Kovalev's last 28 months were: Blake Caperello, Bernard Hopkins, Jean Pascal 2x, Nadjiib Mohammedi, Isaac Chilemba, and Andre Ward.
Outside of Caperello & Mohammedi (who were both on similar levels to the opposition Stevenson has faced in the majority of his fights over the last 31 months), Pascal 2x, B-Hop, and SOG were amongst the fights he had. Stevenson's best 2 opponents were Fonfara and Thomas Williams Jr. For me, the lack of activity combined with the quality he's facing when he DOES get in the ring, plus his stamina problems, and the fact he's only getting older.....I think he's losing those belts as soon as he faces either of the 3 (SOG, KKKovalev, Beterbeast)
That's another thing I forgot...he doesn't have good defense himself either. SOG and Kovalev would kill him from the outside.
Im not going to argue this, bc it's true. Struggleman could bomb Kovalev straight out of there and Beterbeast is all "potential" at this point...I've just seen enough to say, imo, that Beterbeast is legit. I can understand the hesitation from others on him though.