In that sense, if Dembele will be our best attacker in the future, sad times are infront of us.
Dembele has to improve his finishing. He is 23yo, but has lost 3 years of development, so basically he is just 20yo in terms of experience. We can see that he is developing, especially when it comes to tactical awareness, helping his full back with defending and losing the ball less often through unnecessary mistakes and lack of concentration.
We can say that Dembele is not a natural goalscorer though. He can improve this part of his game, but he'll never be a 20+ goals/season player on a regular basis (tbf wingers who regularly do are very rare). His best season in terms of scoring has been his first as a senior in France, 12 goals and 5 assists in 29 games for Stade Rennes. For Barcelona, he has scored 25 goals and 20 assists in 98 games, so basically he contributes to a goal in less than every second game.
Ansu, on the other hand, is showing way more traits of being a regular goalscorer one day. His finishing is very sharp already and he is also good at getting himself into goalscoring positions. Back to Dembele, we shouldn't reduce him to numbers. He is doing the team a favour by creating danger, tying defenders and therefore creating freedom for teammates. Also, he is good at carrying the ball forward.
I agree with you that Dembele can't be the best attacker in the future and you don't win a CL with him as leader. He doesn't need to be though. Everybody here will agree that we desperately need a new #9 and once we got one Dembele will become more efficient too.
With Trincao, we will see. It is too early to draw a conclusion. He can be a flop, but also become good as things stand of now. Also, he is young. Even if he turns out not good enough for Barca he still will have a decent resale value in summer of 2022.