Everyone has their price. The problem for Liverpool is that nobody has any money or would meet that price. That said I agree they should keep both of them, especially Salah, but that also depends on the players themselves.
What Liverpool need to really consider though is getting some new blood and young blood at that. They have a lot of players at around the same age and this side will age pretty quickly:
Salah 28
Mane 28
Alisson 28
Firmino 29
Thiago 29
Matip 29
Van Dijk 29
Wijnaldum 30
Henderson 30
Consider those players' respective ages and the rest of the squad left after them. IMO that leaves them 2-3 years to reinforce the team with an eye to the future. Plenty of time, but they need to get it right too. Otherwise what could have been, and it looks like they could be at that stage, will turn into a flash in the pan.
I agree that their window for top level is closing. That's clearly true. They're not the favorite for the EPL, and they won't be for the CL. I think it's likely they'll be competitive, but I think as long as City still spends like City, and keeps Pep, they will be the team to beat in England. Yet, Liverpool will be better next season. Maybe not last season better, but they should give teams a run. Some will depend on this summer, and they are getting older, though, both keepers and central defenders have been able to be pretty effective, even into their 30s. For Virgil, this injury will certainly be a question, and they have to develop something next to him, but it's not completely over. I think Alisson can have a career of top level for at least 5-6 seasons without too much concern. Will he be one of the best 3 or 4 in Europe? No I don't think so. But a top 10 guy? Sure.
Additionally, while I think the attack will need a refresh (in my opinion Firmino was probably set to make way or at a minimum see his minutes change) and I think there was justifiable hope that a big ticket purchase would have been feasible this summer. I don't know that the money will be there, even if by some miracle the CL / Europa is, so, that might not be the option it was. That really will change things a bit this summer if they don't have the same funding they thought they would.
I will say, that while a lot of their top level talent is getting older, there are some parts in the squad that are just entering peak, as much as others might be winding down. Jota, Fabinho, TAA, Robertson, Gomez (no matter how overrated he might be) should be entering their peak. I do question if some of the miles on Robertson will catch up, but he should still be a quality option. If you phase out Firmino, add in a quality midfielder to the mix (in the expectation that Henderson is getting older, Wijnaldum wants to leave), along with Jones and Fabinho, and that's still a pretty young nucleus. It's a lot easier to do if you can get ANYTHING out of Naby Keita or Oxlade-Chamberlain (jury is completely out on that, and I wouldn't wager on it). But I think the reason Gini is willing to walk, and the club doesn't seem that upset about it is the understanding that they need at least one or two new central midfielders to eat up some minutes (hard ones at that). Jones coming good will help in that process, but I don't think he is ready for those miles.
As for the attack, which is certainly the issue, I don't have problems with going again with Mane and Salah, and using both until they decide they want to leave. It's entirely possible that either player could have 3-4+ seasons of top level football, and certainly, the cost to bring in something equivalent that will be better is difficult to do. The club wouldn't get their value for them (well... maybe, but I think in COVID, they value them more than they're worth) and as long as they're happy, I wouldn't make a move. I think both Mane and Salah have at least a couple seasons where they're extremely good players, though, for me, a lot of this season is less on them, and more the fact that they've got nothing to push them. When Jota was healthy, it was going better, and the club was firing. Sure, Salah might be mad to get yanked, but work your ass off, and you keep the young, hungry player on the bench. Sure, you're a superstar, but there needs to be some hunger for it. I do wonder if one or the other wants a change in scenery. I've always rightly felt at some point, the climate of Madrid or Barca is a calling for most every top player, and it might make sense to see if a deal can be done for their benefit. I don't think Barca have anywhere near the money to get it done (though, I guess if Messi is off the books, there might be a few more dollars to go around), though honestly, I think for Liverpool, it's less what they would get, and more, making a move that they would feel like could help. Letting Mane or Salah go doesn't help you, but letting one go, while freeing up money, while getting younger and maybe a bit more hungry, would be a bit of a refresh at least.
I also think Harvey Elliott is going to be really good. Though, that's a wait and see, and I don't think he's ready to be a top contributor on a CL level side, but sooner, rather than later, he could be really good. I've also been on record saying that Curtis Jones is going to save Liverpool 30-40+ in midfielders. If you can get both of those players for the pennies they've invested, they have more money to go and make a massive upgrade to a winger, or a striker if that's where you need to do it. I think you'd know best that it's a lot easier to fill guys around some parts if you can count on developing, or finding some cheaper options, and getting top level players out of it.