Historically, just blindly looking at stats has done very well in comparison to these NHL scouts that get paid millions of dollars. Actually, many years drafting for pure stats with zero concern for everything else has even outperformed said NHL scouts.
What's the most recent draft we can reasonably go back to, 2017? Here, too, scouts spent all year scouting and rationalizing, while being paid millions.
Take the OHL picks for example(forwards, no overagers).
Draft order:
Tippett
Vilardi
Suzuki
Thomas
Frost
Ratcliffe
Robertson
Formenton
Studnicka
Gadjovich
That's where we got by paying millions. Now, blindly statwatching:
Suzuki
Robertson
Tippett
Gadjovich
Thomas
Strome
Frost
Vilardi
Lodnia
Ratcliffe
The big hits are Robertson, Suzuki, Thomas. All top 5 when just statwatching - Robertson is 7th by scouts. Statwatching top 2 was Suzuki and Robertston, scouts got them 3rd and 4th. Now, perhaps there are more absolute busts when just statwatching, but considering the top end it's still safe to say that statwatching outperformed these NHL scouts being paid millions(once again). And we've seen this over and over again, DeBrincat first comes to mind from 2016.
With that said, you also have several other issues with your post. You use Rantanen to make a point, but Rantanen still had much better production than Slafkovsky. You use the argument "Well Kakko produced well so this argument is worthless", but surely you remember that Kakko also scored well internationally, which is a point you're using to push Slafkovsky?
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Now, on Rantanen specifically, Rantanen's skating improved at a remarkable rate soon after getting drafted - far more than I've seen the skating of any Finn his size improve. We could bank on that or something similar happening for Slafkovsky, but by default I would bet against it.
Furthermore, you make the point about TPS and opportunity, but did you actually watch any of the games as you so have to speculate? If so, you'd realize that TPS gave Slafkovsky opportunities - very strong opportunities right after the Olympics, even. But he could not deliver, and hence got put back down in the lineup. Because TPS was a contender(finished second) and they can't have Slafkovsky in a bigger role if their goal is to win.
All this is to say: 1. Your logic is not sound and 2. Stats are important for NHL projections, often outperforming NHL scouts.