The reason for the 5-ft. step is, that combat is not and should not be static. Combat is a dynamic process with lots of movement, where opponents circle each other and try to outmaneuver their adversaries.
With the 5-ft. step, such movement is possible, since it has no serious penalties attached.
If you basically remove the 5-ft. step, then you will have the result, that characters will rarely move from the spot in a fight, things will just be very static.
If you want to remove the inherent advantages for non-meleers, which can use the 5-ft. step to get away from opponents, you could house rule it in the following way, but be aware, that this makes the life of archers and spellcasters a little more difficult, which does not necessarily have to be a bad thing, tho.
House Rule:
A 5-ft. step does not provoke an attack of opportunity from an opponent, if both the square the creature starts in and the square the creature moves to are within the threatened area of the opponent. If the target square is outside the threatened area, then the 5-ft. step provokes just as normal movement.
Or a slightly different approach...
5-ft. step of opportunity:
If a 5-ft. step brings a creature outside of the threatened area of an opponent, that opponent may immediately make a 5-ft. step outside of the normal turn, when the following requirements are met:
1) The opponent threatens the creature after moving to the new square.
2) The opponent has not otherwise moved any distance (including making a 5-ft. step) during the round (or during the last action).
Bye
Thanee