I think Domi playing on Matthews wing the last few games should show enough of why we should probably resign him.
Yes, some of the plays have been a little too telegraphed, but it's also obvious that the Leafs have been overly trying to get Matthews his 60th for the last couple of games. Matthews has had a large quantity of grade A chances in that time, and a lot of them have been created by Max Domi.
I'm not trying to "compare" Marner & Domi, but the reality is, if you're Auston Matthews being paid $13.25m a season, you need to be able to produce big numbers WITHOUT an $11m winger stapled to you. Let's say Domi costs you $4m.... if his presence means Marner can carry sombody else's game, that probably well worth it.
Plus, his versatility to also be a 3rd line centre is pretty useful, especially considering Tavares' decline.
If it were me, come playoff time, drawing up a theoretical playoff lineup, I'd want Marner with Tavares, Matthews with Domi on his wing, and also Nylander away from Marner/Matthews, likely on a line centered by Max Domi.
If "there's not enough Max Domi to go around", that should say something as to whether or not he should be re-signed.