I don't really agree with your interpretation. Even with the big contracts, the Sharks cap situation was mostly manageable in the short and mid-term. The issue they were going to face was that once their players started to decline, there would be no cap mobility. The only player they lost last off season due to the cap was Pavelski, and at least on the ice, it was proven to be the right move as he took a considerable step back.
The Sharks having to rebuild 4+ years down the line was inevitable, unless the cap exploded (which was a possibility). So adding Karlsson to that junk pile of future bad contracts had no negative effect, since they were probably projecting to have a 4 or so year span where they weren't going to be able to compete anyways. The next 1-4 years after acquiring Karlsson were so much more valuable to the Sharks than the 4 years after that, so it made sense that they wouldn't care if Karlsson couldn't live up to the back half of his contract.
Now, of course that hasn't played out. Karlsson in the short term looks like he is done in the first year of the contract, and the COVID-19 situation has destroyed any prospect of the cap rising for another few years. The latter is something no one could have seen coming last off season.
I don't think anybody should write off the prospect of Karlsson bouncing back. He is going to have an extended layoff due to the COVID situation and San Jose not being in the playoffs. He also has apparently never taken off-season training that seriously, so there is something he can fix. With that said, I also think it is fair to consider this Karlsson's new normal until he proves otherwise. 11.5 million under an 81.5 million cap would be a bargain for prime Karlsson. For the Karlsson San Jose got this season, with no prospect of the cap rising anytime soon, the contract is an albatross.
The most interesting story to me would be if San Jose gets two compliance buyouts. Vlasic seems like an obvious buyout to me, but do they use the second one on Karlsson? I don't think they would, but if they are given a year to use the buyout, and he has another bad season, I think it becomes more likely they use it on him.