The problem is how many teams trade away someone as young but still experienced and as good as Fowler? Not a lot of teams do so making it hard to know how much a team will pay. Fowler is the best one you listed and is younger and healthier then another and more experienced then the others you listed in here.
Your saying Ducks are screwed and will get screwed over by a team how about no. Canucks got a 1st and Bonino with Sbisa who they wanted and only Ducks and Blackhawks Kesler would accept a trade to and Blackhawks couldn't afford his cap so it was a 1 team trade option. Teams don't look to totally screw over another team they don't want to ruin possible trade relations in the future. A 2nd and a prospect like that is terrible just terrible even trade deadline trades for good defenseman get better.
The problem is you're treating Fowler like he's a top pairing defenseman, when he's clearly not. He has years of experience in the league, and while that is a good thing, he's had his fair share of mistakes. Like Ola said; you can see good coming out of him, but he has flaws that are going to be exposed in a deep playoff run, especially if he has an injury or two nagging him in that time.
Kesler was a 1b/2nd line center who had come off his worst season, I remember people were saying it would be a 1st+Theodore+ to get Kesler, and a lot of people were surprised at how little the Canucks got. Realistically, the Canucks got McCann, Bonino, and Sbisa - only one of which who is still on the team 2 years later. The others were packaged as part of other deals.
In fact, going back I see proposals of Kesler for Vatanen, Etem (Who at the time had better value than now), Ottawa's 1st and Anaheim's 1st. A lot of Canucks fans were angry at the value they got for Kesler, who on most teams could have been a Number 1 center.
You don't look to screw over the other team - You look to get the best possible deal for your team. Why would you pay $1000 when you can pay $500 tomorrow. If Anaheim is unwilling to except what other teams are willing to pay, then that's their prerogative. But I'd wager than Murray is desperate enough to sign Lindholm and Rakell, and moving a contract like Fowler's for a pick+prospect clears up all that space.
Not many teams can take on a 4mil contract that goes for the next two seasons, there are a lot less teams in that running then you think. Edmonton, New Jersey, Florida, and Carolina are all potential teams that have enough cap space at the moment, with Edmonton being the only one that may be strapped for cash with RFA signings for McDavid and Draisaitl coming up in the time Fowler will want a new contract.
Detroit and Boston, the two teams you listed as potential trade partners when you mentioned Tatar and Spooner, both will be strapped for cap space in the coming years.
Detroit is already at the ceiling, and would be looking to shed cap, rather than increase it like a Tatar for Fowler trade would do, so they're not a good trade partner for Anaheim.
Boston has contracts for Pastrnak, Spooner, and Morrow expiring next season that they'll need cap space for. A deal of Spooner for Fowler straight up would make it difficult to give Pastrnak and Morrow bridge contracts given the space they'd have remaining.
The teams I listed; New Jersey, Florida, and Carolina are all in different stages Florida is trying to make the playoffs, New Jersey is in a bit of a retool, and Carolina is trying to rebuild. Caroling probably doesn't want Fowler, young as he is, if it means they have to give up more of the future. But Florida and New Jersey may want Fowler, so a trade could happen with either of them. Trading away a pick+prospect to Anaheim for a top 4 defenseman would make their teams better without giving away a roster player.
There are a few other teams I haven't mentioned who would be interested in Fowler and have the space currently, but they have their own players in need of signing, or have contracts coming up next year that would be thrown off by picking up a contract like Fowler's.
So you see how the 10 teams that would be interested in Fowler have dwindled down to 2 or 3, causing less bidding between the two and further driving down the price of any would be trades.