I don't think the comparisons to past teams like the Red Wings, Penguins and Caps are entirely valid.
Many of those teams were operating in a different financial universe than the one the NHL has now. A key difference is that the 'cap circumvention' contracts were used to sign players to lower cap hits than you can under the current CBA. Therefore, being able to bring up young players on ELCs who can contribute is more critical now than it was in the past. The recent Penguin cup teams demonstrated how important this is with guys like Sheary, Rust, Guentzel, etc. Of course, many of those guys were drafted with late round picks, which goes to show it doesn't really matter where the young guys on ELCs come from as long as you find them. Perhaps the Leafs bringing in top players from Europe to play can fill that same role; however, first round picks are still a more likely way to find those guys that the late round picks the Pens did it with.
I also agree with a point already made that trading a first rounder for an impact guy like Kessel is very different from trading for a rental at the deadline who will play a couple of months for you.
If you are a contender and trading your first rounder could make the difference in bringing the cup here, you have to seriously consider it. We won't know where we are on this front until much later in the season. Last year, the guy we needed was already in our system in Travis Dermott. Perhaps something similar happens this year, where by mid-season we have someone on the Marlies who is obviously ready to step in. I don't really have any evidence, but my gut is that this front office will be averse to trading the first rounder unless it's for a true impact player who is not just a rental.