Why would they take out their only viable ruck though?
Because he's not their only viable ruck. He's arguably not even a better ruck than Campbell and Heath is also pretty ready for AFL games and both are pure rucks.
Marshall is much better than either around the ground but he's also a hell of a lot better as a forward and they were using Mitch Owens as a KPF and it flat out wasn't working.
Basically Marshall forward does make a lot of sense, I don't think he will, Lyons has always valued the ruck (albeit the actual ruck not around the ground skills) and I think Marshall wants to play ruck and keeping one of your only elite talents happy is important.
I don't think anyone is really fading him, he's a mediocre pick in a field of bad picks, there's definitely very good and valid cases to not take him just as there are some good cases to take him. If people value durability, he ranks very poorly, proven scoring history he's also weak, job security he's in the middle. Different coaches focus on different priorities. Marshall only made the first draft of mine last weekend for example because he fails badly on durability which given I don't want to use trades on the rucks is a genuine issue.
I'd be more worried by anyone who has locked any ruck in at this point than someone who isn't sure, if you've locked any of these rucks I'd question your selection criteria and what is missing from it that you are at that spot because I can't see anything resembling locks. Dunkley is the only absolute lock I see right now, Yeo is if he stays fit. I'd say right now everyone else should still be on the probable list.