If Maddie died in the apartment we still have the problem of how the body was disposed of. If Jane Tanner did spot a man carrying her away she didn't recognize Christian Brückner as the perpetrator.
If Maddie was indeed abducted, the perpetrator could have been someone other than Bruckner.
I agree, if the McCanns were guilty there is no plausible explanation for how they could have got rid of the body.
Plus they would be breathing a sigh of relief having got away with murder/ manslaughter, rather than continuously keeping the case in the spot light.