The problem is, you are assuming he get's through 4 or 5 games before disaster strikes. What if he doesn't? Given his history, there is a more than reasonable chance he'll miss a game in the first 5 rounds, and without cover it's a poor quality trade or a donut.
As Footyrulz points out in his thread,
The ruck/forward saga, it's not just Sandi that needs cover, as not many good Rucks play 22 games. He's just an obvious example because of a) his history and b) his popularity this year.
Imagine getting caught need ing to sideways trade Sandi after 2 or 3 rounds? Who could you get that isn't going to be a poor replacement at his price? I think if you have Sandi you would be taking too big of a risk, if you don't have cover or a good exit plan. As Footyrulz says, that could apply to most Rucks, but with the people like Sandi, Dawson and HMac, you are more likely to be saying "Damn it! I should have known I needed cover!"