Emre Can
Can is one of the few players on this list whose value spiked after leaving Liverpool- he peaked at €30 million as a Red, and €40 million with Juventus.
Liverpool bought him for a scant €12 million in 2014, and based on some of the performances he's put in since leaving the club, they probably could have turned a nice profit.
Can's Liverpool tenure was just from his age 20 to 23 seasons, so he had a whole career in front of him and likely could have been moved for more than he was worth, relative to what Liverpool were getting out of him.
He left in 2018, just before the trophies started flowing, so it would have been fascinating to see how his valuation would have been impacted, as we've discussed with other players on the squad at that time.
Adam Lallana
Simply put, Lallana should have been sold. After his market value peaked in 2018, he never again started more than five Premier League games in a season for Liverpool, and unfortunately wasn't much of a key piece in any of Liverpool's title runs.
Liverpool initially paid €31 million for the midfielder, and could have done well to recoup at least a portion of that fee at some point later. He'll always have Norwich though, eh?
Conclusion
With so many free transfers, there's bound to be at least some amount of missed value. That being said, with few exceptions, the missed opportunities haven't been too drastic, and Liverpool have done a good job of getting the most out of the players on their squads.
In general, the increased continuity and stability provided by hanging on to proven players has probably been worth the missed income, thus more or less justifying this overarching transfer strategy.