The "Eggshell Skull Principle" is relevant here. This principle states that an aggressor takes the victim according to the victim's individual circumstances. If I king-hit a random bloke but it turns out he isn't great at getting punched and he dies, that's on me. Likewise, maybe a player who is hard as a cat's head like Joel Selwood or Anthony Miles would bounce up from an illegal hit like that, but that DOESN'T mean it's OK to deliver a head-high bump to Jack Martin.
I think the Sandi decision was fair, I hope for consistency for the rest of the year.
I do want to point out that the principle you have stated is not always followed by the tribunal. In 2014, I think, Steve Johnson put his knee into the Roos Scott Thompson when he was on the ground. It clearly caused Thompson pain. With the assistance of Thompson and brotherly love, Geelong argued that Thompson had a rib problem and that was why it was painful. The tribunal then reversed the MRP decision. Again, in 2014, just before finals, Brent Harvey hit Joel Selwood high, causing him to bleed and leave the ground. Again, with the assistance of brotherly love, the Roos argued that Selwood is an easy bleeder. The tribunal accepted this argument. So you don't take the victim as you find him, it depends on how the process wants to find on the day.