Marowak is taking a drop in use because it was one of Genesect's more consistent checks.
Doesn't necessarily mean Marowak is any less effective against Magearna, and while it lost a significant boon with Genesect, it still is a pretty anti-meta tank that finds a role for checking things, or at least pressuring them to consider particular coverage moves (like EQ on Metagross), Magearna being one among that list. Any Magearna set without Shadow Ball is checked decently by Alolan Marowak, right down to an inability to Volt Switch, albeit sets with LO (or specs if that's a thing somewhere) can put a noticeable dent in Marowak. These calcs just use two spreads, one SpD oriented and one more "standard" from the Alolawak QC thread against Modest Max HP Magearna (not sure how much speed it runs, wanted to check most bulk for worst case scenario). Needs to also be careful about switching into Twinkle Tackle on Fairium-Z set, as that won't drop Magearna's SpA and that + Fleur Cannon KO's Alolawak (with any prior damage or Rocks for the SpD set) before it dents back with Fire Punch.
252+ SpA Life Orb Magearna Fleur Cannon vs. 248 HP / 164+ SpD Marowak-Alola: 136-162 (42.1 - 50.1%) -- 0.4% chance to 2HKO
252+ SpA Life Orb Magearna Fleur Cannon vs. 248 HP / 0 SpD Marowak-Alola: 182-214 (56.3 - 66.2%) -- guaranteed 2HKO
252+ SpA Magearna Fleur Cannon vs. 248 HP / 0 SpD Marowak-Alola: 140-165 (43.3 - 51%) -- 5.9% chance to 2HKO
252+ SpA Magearna Twinkle Tackle vs. 248 HP / 0 SpD Marowak-Alola: 210-248 (65 - 76.7%) -- guaranteed 2HKO
96 Atk Thick Club Marowak-Alola Fire Punch vs. 252 HP / 0 Def Magearna: 270-318 (74.1 - 87.3%) -- guaranteed 2HKO
Some dicey situations, but the match up favors Alolawak enough that I'd call it a decent check. Being unable to Volt Switch is definitely a point of note to stop Momentum grabs, and neutral Shadow Bone puts a good dent in non-resists that aren't outright Physical walls, while also being pretty dangerous when two of OU's S-Ranks are Psychic types (one of them also afraid of Fire Punch)
96 Atk Thick Club Marowak-Alola Shadow Bone vs. 252 HP / 0 Def Celesteela: 166-196 (41.7 - 49.2%) -- guaranteed 3HKO
96 Atk Thick Club Marowak-Alola Shadow Bone vs. 0 HP / 4 Def Metagross-Mega: 240-284 (79.7 - 94.3%) -- guaranteed 2HKO
96 Atk Thick Club Marowak-Alola Fire Punch vs. 0 HP / 4 Def Metagross-Mega: 212-252 (70.4 - 83.7%) -- guaranteed 2HKO
96 Atk Thick Club Marowak-Alola Shadow Bone vs. 0 HP / 4 Def Tapu Lele: 432-510 (153.7 - 181.4%) -- guaranteed OHKO
Not gonna be the #2 in usage it was while Genesect was here and Pheromosa was all the rage, but it's still a mon that should be acknowledged if not extremely heavily prepared for when building a team with a mon like Magearna.