If he walks, they're never going to replace Landeskog 1-for-1 or come close to it. It'll be about adding multiple complimentary pieces.
If we know two things about Colorado's front office it's that the pro scouting is excellent and Sakic gets great value in trades. The most expensive UFA Sakic has signed in the last five years was depth defenseman Ian Cole ($4.25 x 3). In that same time he's traded for multiple notable pieces (Kadri, Saad), which includes several RFA's (Burakovsky, Grubauer, Toews, and now Maltsev - obviously he's a depth piece though).
This could be the year the Avs make a free agency splash, but I think Sakic will be active in the trade market. Particularly if Grubauer walks, I expect them to trade for s goalie rather than sign one.