Refs have to 'retire' at an early age because of the physical demands yet their knowledge & experience then goes to waste. Why can't these be used as the 'ref in the stand' with access to screens? they can then be miked up to the onfield ref to act as 'assitance' with the onfield ref still having the last word. this might stop some of decisions which, at full speed seem fine but on review are flawed, like B'muffs pen.
Because the laws are mostly "in the opinion of the referee...", the referee may simply have a different opinion to the assistant in the stands (we frequently see commentators and "expert" analysts disagree with each other after looking at replays, so two refs disagreeing doesn't necessarily mean one is right or wrong, just different opinions of the incident) which undermines the laws of the game, or they would have to be completely re-written to remove any interpretation from the laws.
The ref on the field can gauge his assistant's point of view and judge whether the linesman has a better view than he does and can overrule them based on that, he can't judge that difference with a video ref and know whether it is simply different interpretations or if that video replay showed something the ref missed.