Coutinho definitely could have done better with different manager, Dembele is plagued with injuries, so he wouldn't have developed even under better manager, but what would change for Griezmann if we, let's say get Koeman? He is not a pure striker, so he still need to have a striker next to him, if he plays with a striker, then we'll have the same situation next season again, he'll be stuck on that left wing without having skills to be a winger.
We never should've gotten Griezmann. He was unwanted by the players (not that that should matter) from the start, but even more importantly, like you said, it should matter if he doesn't fit into the tactics, in fact that's all that should matter). And Griezmann, even though we know he's world class and we've seen it, that still doesn't make him nor a 9, and definitely not a winger. He's basically a more attacking Coutinho, but even a worse fit because he's left footed, and since LW is the only attacking position in the starting lineup up for grabs right now, and with Messi and Dembele already being left footed too, you know you should be looking for a right footed LW, who is maybe able to play striker too, when Messi moves back to midfield and we basically play 4-4-2.
And we all knew Suarez wouldn't be benched as long as he's here, considering he's Messi's best pal and 2nd highest paid player. We should've just stuck with Suarez (which we're doing now anyway) and wait and try and go for Mbappe or some new wonder kid next summer while this season just use what we got, play Dembele regularly, while promoting Fati full time and calling up Abel Ruiz for those few games Suarez actually doesn't play, since you can basically count those on one hand, there was no point in getting an outsider until we're truly ready to not only bench, but ship Suarez off altogether. Maybe we can try and swap one of Suarez or Griezmann and some money for Mbappe next summer anyway.
As for Dembele, I agree his work ethic and taking care of himself could use some work, but it all, even the attitude, comes down to being in touch with the players and even more especially the training regime. The team is running 10% less/per game than our opponents, and it all stems from not giving a fuck about physical preparation. Not to mention Valverde having no relationship with the players outside of the mandatory. When you see Klopp smiling and hugging every one of his players, greeting them when they returning to preseason training. Or Pep and his mandatory team dinners and really being attentive to his players, often having one on one meeting with them to solidify that they're happy to be at the club and with the role on squad. And still teaching his players new things every day in training instead of just doing same old same old exercises, basically telling them ''go out there and do what you want, cause I don't have anything to teach you anyway.'' So don't tell me that under Klopp or even more so Pep, Dembele would be as fragile and injury prone as he is right now. You forget, he wasn't injury prone at Rennes or Dortmund, yet you're treating him like he's Vermaelen, who has been injured longer than fit at pretty much every club he's played at. It all comes down to being devoted to the team and to the club 100%, not just physically, but mentally as well. It all contributes to players staying fit and focused, actually WANTING to stay fit and ready so that they can go out there on the pitch and enjoy themselves. While in the state that we are right now, I wouldn't be surprised if players actually get relieved when they get injured so they don't have to go with the team (especially on away trips) and get destroyed by the media after performances like today.
Under Pep, Dembele would've been exactly what Sterling is these days: a world class winger with lungs to track back all the time and fight for every damn ball.