I'm absolutely aware that you can (should) sell high on players when your team is in a situation like the Spurs'. I myself was literally advocating for selling high on Dejounte like two weeks ago

. What you don't seem to understand is that, unlike Dejounte, there's not bound to be a lot of interest on the trade market for Jakob, so the opportunity cost of selling off on him (and get maybe a late FRP and meh prospect, if even) isn't really worth the loss of the glue holding the Spurs' defense in place night-in, night-out. The same can be said for Dejounte's presence on the other side of the ball, btw, which is why I was bittersweet on selling him off, but in that case, his gaudy stats and archetypical value made the Opp cost ratio much better.
Valanciunas wouldn't be a "huge upgrade" at C, FWIW. He's marginally better - significantly better on offense, but considerably worse on defense, which ends up cancelling itself out,
unless you have the personnel to shore up one weakness or the other (the Spurs, as I've said, have neither). So why even bother wasting resources (because you can bet the Pels aren't going to happily drop their 18ppg, perfect-Zion-pairing center for free) for a marginal upgrade (just so you know, and I know PER sucks as a catch-all but it's relevant in this case, Jonas' PER is 1.0 whilst Jak's is 0.4), when the team has much bigger holes to plug?
Again, this is what the "trade Jakob" camp will never get. I can present this to you any way you want - but at the end of the day, Jak has "no dog" to you, and that's unappealable and beyond reasoning. He just Has To Go.