Ferland does not save the leafs cap space, he would make it impossible for them to acrue cap space for the deadline. He is significantly negative value to a cap team trying to compete. And schenn is not needed or wanted by the leafs.
So on top of that, just way too much for one season of miller

What’s the point of trading Ferland when he isn’t playing anymore and we can put him on LTIR for free rather than trading?

Arizona needs no help getting to the cap floor, as they are within $2.25 million of it with Lawson Crouse and Barrett Hayton yet to re-sign plus a backup goalie to add. And that's with sending a forward on an ELC down to the AHL, so they're really not interested in taking on Dickinson.

He wanted to make that trade more enticing for the Leafs by giving them LTIR but Ferland's hit is so small but I guess between the savings on Miller and Schenn, they would get over 5m saved but ofc they can't accrue any cap space so might not be worth it anyways.