That is not what happened. For one reason or another, as from the very start of the season, Salah was utilized in a completely different role than last year, when he thrived. He was shifted out wide right and remained there almost exclusively or for large parts of the games. He didn't do enough with the ball when he had possession or didn't convert all of his chances, so he's partly to blame, but that was never his game anyway. He has had loads of off-games which he still decided with one brilliant move or goal, those just didn't come early in the season - whether that's down to his positional change or to himself is up in the air. For me, the drop off is too big in too short a time to be completely atrributed to the "his legs have gone" argument.
Imo it's much more likely that Slot wanted to install his own attacking philosophy onto the team, while simultaneously integrating Wirtz and Ekitiké, and failed badly.
I see your point, that Salah’s fall in output needn’t have been as big if he was utilized in the same role (with the same tasks and freedoms) as last year. Sounds plausible, absolutely.
You point out that the philosophy of the coach, and the integration of two (three, if we count Isak) big investments for the future, were at odds with Salahs role from last year, and even if the total (Slot, Salah, Wirtz, Ekitike, Isak pluss the rest) have failed, my question to you would maybe be: How good would Salah have had to be this year for it to make it worthwhile sacrificing the philosophy of the coach and the three biggest forward investments of the club for years to accommodate a tailormade role for a 33 old forward?
Take a player like Christiano Ronaldo, who was top scorer and player of the year at United when he returned, but whose accomodation into the team was probably the main factor in our drop from title outsider to mid table candidate. But a better example is Ronaldo at Juve, closer to the height of his powers, were it still seems to me that Juve would have been better off investing in a future rather than building a team around a 33 year old. The team that had won six straight leagues, four straight doubles, still won two more leagues with him as top scorer, but forfeited the league in his third season and has not won a league since (six years).
My suspicion is that even if Slot is the wrong choice (he did look the right choice one yeqr bqck, and even if Wirtz, Isak and Ekitike are not the right successors (which they well may be), the idea for Liverpool was the right one: To actively move on from Klopp and Salah in a deliberate manner, rather than trying to csqueeze the last bit of success from it till it fell apart by itself. And if they do that, it implies that Salah must start doing what’s useful for the team more than the team doing what will make the most out of Mo Salah. Many great players have great ego’s and struggle with that shift. For us, I’d say both Ronaldo and Rooney struggled with it, but Ronaldo handled it badly and imploded in a way detrimental to the team, whereas Rooney handled it well and found ways to be useful to the team even if it made himself look a worse player.