This is extremely optimistic.
There’s improvement. And then there’s improvement that makes a difference. Hamonic on a pair instead of Zaitsev doesn’t take that pair to the next level. And Zaitsev on the third pair instead of anyone else currently slotted to play there certainly doesn’t even improve it it actually makes it worse.
Sanderson needs to be great. Like truly great for that defence to stand a chance
I'd argue you're actually the one being extremely pessimistic.
Personally, I saw Hamonic as a substantial upgrade over Zaitsev. He handles the puck much better, and is just as good if not better winning puck battles and on the PK. Hamonic also doesn't make the really questionable decisions defending two on ones that Zaitsev does.
Wrt the 3rd pair, we went most of the year with Mete, MDZ, J.Brown, and Heatherington on it. Zaitsev is an upgrade over last year, but if he's not an upgrade over who you think is slatted to play there (Thomson, JBD or Holden on his offside I guess), well that's still a good thing, and an upgrade over last year. If we have Zaitsev stepping in when someone gets hurt instead of Heatherington, that's good not bad.
As for Sanderson, we can look at comparable d over the years to get an idea of what kind of impact he could have.
Take a look at guys coming out of college who were similarly touted, McAvoy, Hughes, Makar Wierenski, Hanninfan, Trouba, all these guys came in at a similar age or younger and had immediate success in top 4 roles. He's going to be fine, his skillset is custom made for today's game.