Sure, he helps a lot right now, but if they keep him, they will get just a few more years of good play from him, then he will be too old. However, he is such a huge name that if they sold him for example to some Saudi team, then they would make enough money to buy someone else who would be able to give them maybe 10 years, that is a lot better. They are on the verge of missing out, maybe they already did, this summer they should definitely sell him for a good money, if not, then it will be too late.
Salah will probably be sold to a Saudi Arabian team this summer. A Saudi Arabian team made an offer for him after the summer transfer window was closed as well by the way. But Liverpool didn't accept the offer despite its being a gigantic one. Because it was out of transfer window for Liverpool so they wouldn't be able to transfer anyone to fill his place right away.
But this summer Liverpool could be open for offers. While they have an opportunity to make a lot of money by his transfer they should do this. Salah could also be keen on leaving depending on the amount of contract offer.