I think that's absolutely true. Tolvanen definitely brings more than Sprong and fits better in terms of his overall game. I'd also agree that Donato is a better 4th line/role player fit as he does bring a little more of a 2 way game along with high effort.
As we grow/develop/evolve we have to look at individual roles more and one dimensional scorers don't really fit on what you want out of a 4th line. We need something more like the Vegas unit ideally.
Sprong is kind of similar to Olafsson in Buffalo. Another over 20 goal scorer who got moved to the edges of the line up to let the younger guys develop (Peterka, Quinn). The spots are there for Kartye, Wright, etc. to earn.
I am very hopeful about the Kraken development pipeline as far as forwards go. Kartye is very likely to start the season in the NHL. Wright probably needs more time in the AHL. Not just that, I would not be shocked to see one of Melanson or Winterton get a look. Especially Melanson, with his NHL-style game and physicality. Winterton rebounded big time in the playoffs leading all of his league in points, though I think he needs some time in the AHL first.
Then we also have Morrison, who, fingers crossed, could be this season's Kartye.
Outside of what you mentioned about Sprong, a big issue is that he is an arbitration-eligible RFA. What he would get rewarded as a 20G scorer would likely be too much, even for a one-year contract. It may be in his best interests to look for a cheap-ish multi-year deal elsewhere. Ideally, if we could do a sign-and-trade or trade his rights, it would be great, but I don't think that happens.