The problem is that he can't be sent down to the AHL because he's waiver eligible, and he needs more time in the AHL to round out the rest of his game. He's about in the worst spot possible for a prospect, he's waiver eligible while needing more time to develop on a win-now team that isn't focused on developing young players. Sullivan doesn't like his game (and I don't blame him), which just makes it even worse for him. Sprong's value is almost completely down the drain because of his waiver eligibility, which is a screw up that dates back to 2015.
I'd 1000% do DeSmith and Sheahan for Talbot and Khaira, I just don't see why the Oilers would do that and it seems really bad in value for them. Even with Talbot struggling this year, he still has a track record of performing in a starter's workload. His value may be lower due to his struggles, but I don't think he's valueless. Even with DeSmith having a really good start to his NHL career, his value is like a 4th round pick based on what other backup goalies have been traded for.
In this trade, I'd probably say Sheahan, DeSmith and Khaira are both worth mid to late round picks (3rd or later) and Sprong is worth a middle round pick (2nd or 3rd). I may just be overvaluing Talbot, but I feel like a 2nd and 4th for Talbot at 50% isn't a ridiculous suggestion.