Yes, it will be sad enough for Liverpool fans if Selah Salah and Manya Mane leave the club at the same time. Still, it can be said that they made the greatest contribution to the club's entry into the Champions League final.
I disagree with some of your points. Mane and Salah only won one champions league together at LiVARpool, so saying they made one of the greatest contributions to LiVARpool's European football is incorrect; LiVARpool were already the most successful English club in the champions league prior to the Mane-Salah era. I would have agreed if you said they gave LivARpool their first EPL trophy.
However, I don't think that the departure of both players will be a disaster for the club, but it may reduce the power that Klopp has in the formation of the squad. As for Salah, it is really strange that the parties cannot agree until the end of the season itself, although Salah himself seems to have curled that he is not going to leave Liverpool.
Yes, no player is bigger than a club, but losing two of your best players without a proper replacement will have a significant impact on the team, especially if the team is built around those players. Additionally, Salah has not stated anything about his future, but if both parties fail to agree on something this summer, Salah will walk alone next summer.