Oh boy would I hate to have to play like that! I would immediately counter that initiative accounts for the fact that my actions during the round are staggered from my allies and enemies alike. While I am finishing up my combat maneuver (re-readying my sword, raising my shield, getting my fingers ready to cast the next spell, etc.) others are in the middle of their maneuver. Seeing their outcome, whether failed or successful, it would seem "logical" that I could adjust my tactics based upon what happened. Having simultaneous declarations, while maybe appearing more realistic, will do nothing for the "game" aspect of it all other than to slow combat down and cause frustration. "Oh man, my declared target is dead. Great, a wasted action."
I know there were a few computer games that were like this. But I think they were wargames representing large armies. This was all before Real Time Strategy games came about and eliminated the need to simulate initiative.