Surely you can see the difference between pursuing a washed out Karlsson and paying high end assets and getting a 24 year old franchise player in his prime right? Or maybe you can't you can never tell in these parts. Honestly I really have no idea what Eichel gets in a trade right now, but I have no doubt there will be a lot of potential suitors.
First I'd argue that many didn't think Karlsson was washed out when he was being shopped, either. If San Jose did, they certainly wouldn't have given up what they did in the trade and then signed him to the deal that they gave him.
Second, whether Eichel returns exactly to form or not, this doesn't make San Jose a contender. They get Eichel for 5 years at $10M. For four of those five years, they've got $26.5M tied up in three immovable contracts (Burns is still pretty good, but nobody's touching that deal due to his age) and another $5.75M in Jones, who's been abysmal, for three years.
Realistically, they don't have the cap structure to make it work. They'd be a fringe playoff team at best that doesn't get close to a Cup during Eichel's contract, which means that they'd face the very real possibility of selling off massive assets for Eichel, not come close to a Cup during the time he's there, and then watch him leave in UFA because of it.
Honestly the best option for San Jose is selling what they can and riding out those contracts while they stock the shelves. They weren't exactly in good shape before the Karlsson trade, but the trading for and signing of Karlsson has pretty badly screwed them.