You've been advocating that we need to unload Kessel either this offseason or next. The most cautious thing to do, and possibly the smartest, is just to move Rust to LW for this year (or fill in for any injured top 9 RW's) and see what we've got in Sprong. That helps to inform our decision on Kessel for next year.
You also end up a worse team in the short run by doing this and you force Rust into a position that he's less effective in. People say that they can't trade Kessel because they'll become worse next season, but they also advocate keeping Sprong and making the team worse? Doesn't make sense to me.
Yeah, we do need a better fit on LW. But right now our depth is Guentzel, Hagelin, and possibly Rust/Sheary on LW. And we only really need a 3rd wheel on that side with the RW's we've got. We should be able to find that without giving up Sprong.
If you're not going to give up Sprong or another similarly high value asset, you're not getting an upgrade on what you have already. That LW group is extremely mediocre, and you'll likely see it take another hit with trading one of Hagelin or Sheary (probably Sheary). LW is probably the clearest weakness of the team in terms of position (so comparing LW to C, LD, RD and such), you're not going to improve it any tangible amount by just poking at it.
Now, if Rust was actually as effective on LW as he is on RW, you'd be perfect on LW. But he's not. One of Sprong, Kessel or Rust needs to go out for an actual LW improvement, you either do that or your LW depth is an issue.
And we don't absolutely need that LW fit until the trade deadline next year. Although Domi is a good player, his defensive game isn't exactly conducive to covering for a Geno/Phil pairing. I'm just not convinced that's the guy you need for fit, and we'd be paying an awfully heavy price with Sprong+ to find out.
A. I don't think you're paying a heavy price on Domi. And even if he doesn't work out as that 3rd wheel to Malkin and Kessel, why is that the only role he can work in? Why not LW with Malkin and Rust? Why not with Sid, where Guentzel slides down to play with Malkin and Kessel?
B. You're also not mentioning the risk that comes with not even knowing if Sprong is a NHL player. I'd rather gamble on Domi, a guy who's firmly established in the NHL, being effective here than Sprong.
C. There's also no guarantee that you're going to be able to pull off an actual LW upgrade for the cost that Domi would probably cost you in the offseason.
I feel like people are either really overrating Sprong or underrating Domi. Domi is only like a year older than Sprong and has proven to be a top-6 player already.