Any fan of any club on the planet would be happy to hear their best player (s) are extending their contract. I think you're reading far too much into this tbh.
I can see Salah going to another club within 18 months, not so sure I see Mane or Firmino going in that time frame, unless we start regressing. Can you see them at Madrid, Barca, City, Bayern, Juventus, PSG? I can't see those clubs spending a bucketload on either.