It's hard to step in the way of a player who needs regular minutes to develop, and everyone knows it, when that player wasn't getting into the side (rightly) because of the depth ahead of him.
Salah had to develop to get where he is today. That took his time at Fiorentina, where he looked alright, and Roma where he looked pretty good. At Roma he learned the confidence to use his speed and his skill to take players on - and he started getting into good goalscoring positions and created a lot of opportunities for other players. He took those skills and developed a more clinical shot with us, because our system puts him in a role that takes these very skills he demonstrated at Roma and tries to constantly utilise them and have him using his pace, skill, and shooting as much as possible. Our front 3 moves around constantly trying to have him run at the weakest gaps in a defense.
But for him to get to be in the shit hot form he's been in this season, he first needed to develop and get the game time he got with Fiorentina and Roma. And then he needed to be put in a system that really allows him to shine.
Chelsea managers are put in a hard spot when it comes to giving promising players time and chances to develop, because the pressure is well and truly on to get immediate results. And it's hard to give a kid who's new to the league and not looking great when you give him chances more chances, even if he does need more time to establish himself and to show what his potential can unfold into... because the evidence has shown they're not ready and when you've got players who aren't ready and players who are ready and you need results - who do you pick?
I don't blame anyone at Chelsea for letting Salah go at the time. He wasn't looking like he'd establish himself and it wasn't safe for a manager to say "go on lad, prove yourself" because that manager doesn't have the time for that. The loans were fair to Chelsea and Fiorentina/Roma and all clubs got to see how the player was after that much development.
The fact that Salah ended up with us and in a system that compliments his strengths really well is sort of besides the point. Who would have predicted Klopp would put that top 3 together at the time of the sale? Nobody. At the time of Salah's sale to Roma he was still honing his skills... and I don't think he's done either tbh, because we saw over the season he became much more clinical. At the time it looked like Chelsea selling a player they bought but didn't need at a fair value to Roma, who had used him and found him to be a good player at a good value price. Fair deal, nothing wrong with that.
Nobody can predict the future.
It just looks much worse when you consider Chelsea also let Lukaku, and KDB leave under really similar conditions... and it does make you wonder whether Chelsea should give managers a bit more time and a bit less pressure for immediate results (which are always important, of course, but not as important as following through on a long term plan that can sustain success for longer). As they could have developed the 3 of them and had an incredible attack.
But at the time... and considering the way Chelsea operate, I don't think there was anything wrong with the decision. The fact that a promising young player you sell can potentially end up being world class or very very good is a risk you take when you buy these players and hope for immediate success.
With Chelsea, the quest for immediate success and not giving managers has worked for them tbh. So there's definitely validity in their strategy. And the counterpoint to the risk of letting these class players go is, clubs that do try to develop players can sometimes go wrong and go on long barren spells. And that's true as well. Both strategies/viewpoints have pros and cons.
Personally, I like seeing players develop at my club and it's cool when they really kick on. Like I remember Lucas Leiva getting absolutely slaughtered when he first joined, and after a while he was a cult hero with us. Jordan Henderson came to us as a pretty shite player on our right hand side. Brendan Rodgers almost swapped him for Clint fucking Dempsey from Fulham and he stayed and wanted to fight for his spot. Now he's captain and when healthy, almost certainly our best central midfielder, and taking us a CL final. And it's very cool to see a player go through all of that with your club.
But on the other hand, chasing after immediate success has given Chelsea fans a lot more big nights more frequently than we've enjoyed in the past fucking decade.
tl;dr - long post