Well, you asked me why we would need to keep Sprong, and the main one would be if we intend to lose Kessel. If we trade Sprong now, and Kessel next offseason, then we go from having awesome RW depth to having Rust and PH as your top 2 RW's.
So we either have to ride out Kessel's deal, or keep Sprong. We can't be trading Sprong for a "fit" fix on LW. I think we can't be trading either until we know what Sprong will be at the NHL level.
I don't see why you have to do that, to be honest. You don't need to have either Kessel or Sprong in the long run, and without either of them, you still have 2 top-6 RWers. They don't need to cling to 4 guys who are either top-6ers or future top-6ers because they'd only have 2 top-6 wingers if you trade 2 of them. You can trade Sprong for a LWer this offseason, trade Kessel for futures next offseason and then sign a 2nd or 3rd line offensive RW and be completely fine.
We're making this way harder than it needs to be. Just package up Sheary plus whatever futures are needed to get our fit on LW, or sign someone and trade Sheary for a pick. You've got guys out there like Comeau, Roussel, Calvert, or Pouliot that could probably be signed for reasonable deals and could slide down to a really good L4 LW if Rust outplays them on LW, and you give up nothing.
A. You're not going to get any sort of upgrade on LW by offering a package starting with Sheary.
B. There is no one in free agency that is an upgrade on what the Penguins have, that the Penguins can afford. Guys like the guys you listed do absolutely nothing to address the Penguins issues on LW, they're all worse than Hagelin and probably worse than Sheary.
The Penguins need another top-6 LWer. Unless you're paying JVR $7 million a season, you're not getting an upgrade on who you have right now in free agency. If the Penguins would trade Sprong for Domi and trade Kessel for futures, they'd have 2 top-6 LWers and 2 top-6 RWers long term and a lot of cap space to work with to fill the rest of the roster. This really just sounds like not wanting to give up your team's best prospect, that's all it really is. You're basically neglecting your actual areas of need in the name of preventing an area of strength from weakening.
Within the next 2 years, the Penguins already questionable LW depth is going to take an even bigger hit when Hagelin leaves in free agency and Sheary is traded for cap relief. Your LW depth becomes horrendous immediately, because it goes from Guentzel to ZAR to nothing. Even with Rust on LW, your LW depth is still really bad. The Penguins have both an immediate need for a LW improvement and an even bigger long term need for LWers. Even with the Penguins trading both Kessel and Sprong for nothing, their RW depth is in a stronger position long term than their LW depth. Sprong for Domi, in this case, would ensure that your top-6 wing spots are all filled with top-6 caliber players.
I have seen no issues with Rust on LW. What am I not seeing?
Rust is dramatically less effective defensively on his off wing, and that's basically his biggest attribute. If anything, they should be moving Sprong to LW, because he already sucks defensively and his biggest attributes would benefit from being on his off wing.