@Jame
You asked for a breakdown on why Sobotka is washed so let's go into the numbers.
Role
In 17-18, Sobotka was a utility forward who served as the energy guy while others drove the play. With Stastny, he was used in two-way roles that shifted slightly towards offense, and was a more generic third line type with Jaskin. While his defensive zone usage is on the heavy side, Sobotka wasn't carrying the weight of a specialist.
Usage
13:55 - 5v5 TOI/Game
1:38 PPTOI/G
1:08 SHTOI/G
46% - ZSR
18.92 - OZS/60
22.06 - DZS/60
Teammates
8th - TOI
1st - CF%
1st - xG%
Competition
7th - TOI
7th - CF%
9th - xG%
Lines
Sobotka-Stastny-Steen
Sobotka-Stastny-Tarasenko
Sobotka-Thompson/Barbashev-Jaskin
Production & Impact
While Sobotka's individual production is uninspiring, you probably aren't targeting him to rack up points. That being said, he was deployed with a suite of effective forwards, so you'd like to see a little more in this department.
The real question with a player like Vlad though, is whether the things that make him a fan favorite actually translate to the ice. Sadly, they do not. Sobotka is a drag in virtually every underlying metric, and things don't get better trying to isolate his impact in one zone or another. There is a consistent trend of the team and linemates generating less and surrendering more with Sobotka on the ice, in both quality and quantity.
While I'll spare you the individual O/D splits, but once you weight for how his linemates defend away from him, Sobotka's drag on xGA notably stands out among regular Blues forwards.
Individual
1.18 - P/60
0.92 - P1/60
0.48 - G/60
On Ice
46.16% - xGF%
49.75% - CF%
47.48% - SCF%
42.86 - GF%
Relative & RelT
-.7.89 - xGFRel
-2.67 - CFRel
-5.54 - SCFRel
-9.68 - GFRel
-7.05 xGRelT
-2.69 CFRelT
-9.26 GFRelT
Verdict
Although Sobotka is clinging to a low-end third line role in terms of individual production, his inability to drive play in any capacity really makes him more of a flexible 10th forward at this stage of the game. He can play up the lineup in various capacities, but it will typically be due to a lack of options or outdated coaching tendencies than ability or impact.
For the Sabres, he only makes sense as a fourth line identity piece to cap off an eventful overhaul of the forward group. Sobotka does nothing to make Eichel, O'Reilly, Mittelstadt, or hypotherical checking center acquisition's lives easier, and should not be viewed as an answer to any of the team's core problems. Hitting people is not a substitute for scoring or preventing goals, especially not for this team.