As for me i think I will prefer one player to go for the other, their combination don't seem to be as expected, Sallah is good quite alright and Mané is skillful as well, but one thing i noticed about Mohammed Sallah was that his performance has dropped drastically unlike before, its on a rare case you will see Sallah not making a goal in any of his match, so its either Sallah go nd leave Mané behind or other way round for them both, i think they should have a rethink in that aspect and i will personally prefer Mané as against Sallah to stay.
You are wrong, even if you think Salah has had a decline in form this season then you seem to have forgotten that Salah is the Premier League's top scorer and top assists this season with 23 goals and 13 assists. No player has consistently scored goals in every game especially in a competition like the Premier League, but I don't think Salah's form has dropped much compared to before.
Actually, I don't think Mane said he wanted to move to another team next season, but it was Liverpool's decision to sell him to Bayern Munich. The combination of Mane and Salah has helped Liverpool a lot, but next season we will not see Mane again in the Premier League.