I don't even think Hagelin is doing BADLY, either. He has picked it up a lot lately after a slow start, actually. But I dunno... while there are plenty on this board who will clutch at their pearls at the notion of a Sprong/Malkin/Kessel line and who will do the "dirty work" etc etc I think there is also something to be said for sheer overwhelming offensive firepower.
Pipe dream, though. I don't think Sprong has ever even taken more than a few shifts at LW. And the coaching staff would probably rather eat glass than make that line happen.
Again... I just hope they find a way to work him in with Geno and Sid here and there. Keep hi confidence up.
We've tried something similar with the GMK line. It's not been bad but it hasn't really justified concentrating that much offensive firepower onto one line, and I think Guentzel is a better complimentary player for Malkin and Kessel than Sprong (who is literally doubling down on Kessel's skill set). Granted, its a little different with Sprong because its not taking an established attacking player away from the rest of the roster and it might be a good way to get him some more comfort, but I'm not sure its a winning move in terms of actual success.
Meanwhile HMK is a beast line. Yes there's a certain level of inefficiency there with Hagelin's finishing ability but it generates so much it doesn't matter. I don't care how many points he picks up when the line is pumping out 3.77 g/60.
And the stuff about doing the dirty work and defence is more than you seem to be suggesting. Hagelin's been a very prominent net front presence on some of Kessel's most recent goals. Hagelin's work buys space - would Sprong do the same thing? Would Sprong's confidence remain as high when the line gets itself in defensive troubles and he ends up being the guy getting isolated and making bad plays and maybe getting scapegoated?
Not to mention that we know Sprong can produce elsewhere in the line up... but getting the most out of Hagelin relies on having him with elite talent. Hagelin with Malkin/Kessel is just a defensive player. Sprong isn't.
I don't want to dismiss the idea out of hand but I see a lot of negatives and not many positives.