If that became onside, football would have to adapt more than to any other law change in history, and not in a good way.
If teams defend deeper then you see more "horseshoe of death" type passing from side to side trying to find a chink in the armour, rather than a fast paced, counter attacking game like this here today, the reason why this game has been good is because both teams defend with a high line and both teams press high. If both teams defend deep there is no press, there is no pace to the game. You want this for more entertainment? It would have the opposite effect.
All we need is automated offside, why they didn't bring that in this season is beyond me.