Problem is that very few people know the value of non-star players on other teams. If a middling player is offered on this forum, at first most don't know his value, but some inevitably say, "we don't want your crap." If offers continue, the rest of this forum pucks up on the idea that he's crap.
Staal has value. Even without anything retained. He is a good top 4 defenseman in his 20s. The salary he got is what teams generally pay for guys like him who hit the UFA. If the Rangers pick up even a million, he will bring back good value.
As you kick and scream that you don't want him, forget his name. Can you use a top 4 defenseman? If so, your team could be in on Staal. You may not want him specifically because his name got trashed and in your mind, he's a borderline NHLer on a $5.7 contract, but he's not. His name got trashed on this forum, but GMs don't go by this forum's opinion.
Forwards are sexier for fans, but teams are always in desperate need of defensemen. You don't sign someone you were going for, someone gets injured during the season, etc and you just blew a hole worth 20 minutes of ice time. Staal can defend against any superstar like a warrior. He used to be one of the best in the league defending, he's still very good at it. Certainly if you play him against second liners, he'll do a great job, but he'll be capable against first liners. All the teams that need that will take him, and pay a price for it.
Why should the Rangers trade him? Because they're on the way down, early in the rebuilding process that will take 6-10 years. No need to keep 29 year olds. That's why Brassard got traded. That's why most fans think Zuccarello, absolutely beloved by fans, should be traded. Klein is well liked, but people want him traded. There's a realization that if you're over 27, you probably won't be around for the Rangers up swing.
You could literally read my post from the very beginning of this thread.
The fact is, while Staal might be a #4, he is well overpaid and has a NMC, one which can't be voided anymore. Combine that with the stagnant salary cap and the expansion draft, and more than half of the teams are suddenly not interested: they can't afford to add him, or they would lose something very significant in the expansion draft. Florida is a good example here: they could even send Bolland back to negate the cap coming with Staal, but when there's an expansion draft coming soon, they would lose Demers. There's no point in adding Staal, if you have to give up a better defenseman a year later for free.
The exact reason which you stated, which is the Rangers being on a downswing, is a valid reason to ship these highly paid players around the 30 year mark out, but that doesn't necessarily mean that every other team looks at those players in the same way. Most teams who are looking for a veteran defenseman have too little cap space, OR their need for flexibility down the road is too big to add someone for 4 (?) more years at almost six million per. You seem to not understand the importance of cap structuring, which is something that GMs have learned to do. That has made moving high AAV contracts tougher.
Here are some examples of trades involving a player being traded WITH a 6m+ contract in place:
Weber - Subban
Hall - Larsson
(E. Staal - 2x2nd, Saarela)
Phaneuf, 4 contracts - Michalek, Greening, Cowen, Lindberg, 2nd
Kessel, 2nd, 2 contracts - 1st, 3rd, Spaling, Harrington, Kapanen
Spezza, Karlsson - 2nd, Paul, Guptill, Chiasson
As you can see, the first one had a 1D going both ways, so it kind of doesn't fit here, but since the parameters are what they are, I included it here. Hall for Larsson doesn't also belong here, since both contracts are stupidly good with no clauses whatsoever. But the bottom four are wonderful examples.
Staal was retained from 8.25m by 50%, and even then, there was only one taker. Being a rental, moving him was easier, and the return was good for the Canes, but that just shows how contract clauses can make life harded for teams trying to trade their players. The Phaneuf deal sent a clear cut top 4 defenseman on an expensive long term deal to Ottawa, for... almost 9 million of dead cap, a pick and a prospect. Kessel had to be retained by a bit, and he got back some good pieces. But again, you'd think that a goalscorer of his caliber would fetch way more. He didn't. He had a NTC, which eventually left the Leafs with just one trading partner. Take it or leave it-situation, the Leafs chose to take an underwhelming return. And finally, Spezza had a 7 million AAV contract in place, and Ottawa got pretty meh pieces back.
Long story short, long, expensive contracts with clauses are very hard to move nowadays. There's no way around it, even if the player is good.