Firstly, we're not really sure of offers, valuations or to what degree there is interest in our players so it's hard to determine what "on the cheap" is as things stand or the extent to which someone is valuable. For all we know, the interest in Clark stems entirely from the fact that Ljinders knows him and it's a one off opportunity to extract a higher price than we might get otherwise.
Secondly, sure, a players value could go up if he does well but it could also drop if he doesn't. There is an element of risk. It's a bit of a gamble. It is what it is, you have to make a judgement call.
Look at what happened with Phillips at the more extreme end of things - we could've sold while his value was high for some money but instead signed him up to a long term contract and he's still here at 27 (wow!) doing nothing. Morton is 21 and has been on loan two years running. Perhaps he doesn't want to go on loan again or if he is willing, wants some reassurances - can we give them to him? Not really.
Lastly, Quansah and Bradley established themselves as first team players last season. Clark, Dann and others got significant minutes. Doak would've been given opportunities had he not gotten injured. I don't know in what world a young player would look at Liverpool - of all the top clubs in the Premier League - and go yeah, no pathway to the first team there. We'll see how things go with Slot but I can't imagine we'll suddenly start abandoning youth promotion as a key strategy. We can already see that a path is being laid out for Nyoni.