If you think back to Klopp's first season, he gave a lot of academy players a chance then too and within 6-18 months most of them got sold too. I think we all fall into the trap of seeing a young player get a chance and probably exaggerate how well they've done because they're an academy player and forget just how difficult it is to reach the standard required to play for Liverpool.
Clark's the only one that's been sold (or likely to be) that wasn't an obvious sale imo and it's impossible to know what Klopp would have done had he still be here. I like Clark but I felt his limit was that of a good solid squad player (he was never going to become Mac Allister for example) and even that was never guaranteed. Where I think Klopp leaving has made a difference is more in the way the midfield sets-up rather than views on youth players - in Klopp's 4-3-3 Clark had a natural position as one of the 8's but with Slot it's more two 6's and a 10 and I'm not sure Clark fits either role.
I guess the guaranteed money on the table for somebody that wouldn't feature much (if at all) for us this season and only had a small chance of establishing himself was too good to turn down and if we do have some sort of clause (matching rights, first refusal etc) then it covers us in the eventuality that he does kick on. I think it's a great move for Clark though. He's going to work with a manager that knows him, wants him and he'll get the game time he needs at this age - if he's going to make it then he's been given the best chance possible.