If both Mane and Salah leave Liverpool next season, Liverpool will face a lot of problems. The Liverpool team is stable and strong. But their performance is largely dependent on the performance of Salah and Mane. So Liverpool should not sell these two players before finding a replacement. Although Liverpool did not win the Champions League and Premier League titles this season, their performances were very consistent. Let's see how Liverpool's management decides.
It is undeniable that Liverpool's strength has won several trophies from the enormous contribution of Salah and Mane. Maybe as you say, if Salah and also Mane 9 (seems to be rumored to Bayern Munich) have been confirmed. Then there will be many vacant positions that are difficult to fill. Liverpool's target next season must be more clear because if in the end can't find a suitable replacement I don't believe Liverpool's performance will exceed the current performance.
I wouldn't say that Salah and Mane leaving Liverpool will be a backward development for the club as the many young players that can fill in for their position and will possibly perform better than both Salah and Mane, I know both players have made good name for the club with their past performance but there is nothing any one can do about players transfers. Since that will give them better opportunities.
We have seen Juventus say the same thing and let Cristiano Ronaldo go and they brought in Dušan Vlahović as their main striker and we have all seen how that went for Juventus.
You can have a lot of talent in the team but at the end of the day, you will always need the experience to win your matches. Muhammad Salah and Mane have done very well for Liverpool and right now it is hard to imagine Liverpool's team without these two players.
But eventually, they will leave and there will be players always available to take their position but whoever comes next will need some time to get settled in the team.