Barbashev has been a replacement level player every year of his career and one of the worst in the league over the past 3. He's not good on offense or defense. Obviously, lots of 4th liners aren't but there's some that are at least decent defenders. Tarasenko shouldn't be considered a mainstay on that 3rd line but it's kind of a Frankenstein to begin with. Sanford can continue the cycle, but isn't great at starting it or converting it to chances. Thomas is best off the rush, but that doesn't help Sanford or Tarasenko much. Maybe Tarasenko can eat 3rd line competition, but he'd need to be a lot stronger along the wall than he has been recently, and Sanford and Thomas aren't the puck hounds he's had success with in his career.
Over the last 3 seasons Barby has paced at 13 goals and 28 points in an
overwhelmingly 4th line role. He hasn't been in the top 10 of Blues forwards in EV TOI per game in any of those 3 seasons and consistently is left in his 4th line role through injuries while other guys are plugged in to fill holes in the top 9. Additionally, our 4th line has pretty consistently been deployed in hugely defensive roles in the last 2+ years. Replacement-level offense is absolutely not 13 goals and 28 points a year
in a defensive oriented, 4th line role playing roughly 10 minutes a night at even strength and almost no PP time. You aren't getting 30+ even strength goals out of a 4th line filled with waiver fodder.
His shot is above NHL average (not replacement level, but the average NHL player across all forwards). He's played 270 NHL games with a career shooting percentage of 17%. His shot is easily his biggest asset and that just so happens to be an asset that is fully ignored by every possession/predictive/advanced model that deems him replacement level or below.
There are plenty of flaws to Barby's game and I don't think he has the vision to be more than a bottom 6 guy who can surprise goalies with his shot. But it is just not true to say that he isn't good offensively
by 4th line standards. He is better in the offensive zone than most replacement level and 4th line level guys. And that is really all you are asking out of your 11th forward when you are comparing your 4th line to others around the league.
Edit: over the past 3 years Barby is in a 10-way tie for 201st among NHL forwards for even strength goals scored. He is a +5 over that stretch, so it isn't like we are bleeding goals when he's been on the ice. I'm not trying to say that he's an amazing player and deserves more time. I think he is well suited to his role. My point is simply that replacement-level guys getting 10 minutes a night in a defense-first role aren't going to chip in 11 even strength goals a year for you. At the end of the day, Barby and Sunny were 66% of a 4th line that was viewed as a major strength of a Cup winning team. Steen played a hell of a big role on that line, but Barby is absolutely a good enough player to not turn a 4th line into an obvious weakness with his presence.