I disagree
A character that is using a weapon two handed for more damage does not gain a great deal from it unless they have at least a +4 Str bonus. To get that bonus, they are giving up a Sheild.
I would have to ask you what exactly led you to the conclusion that 1.5 damage for two hands was too powerful? But for the moment, lets go over your four points:
1st) A two-handed weapon already does significantly more damage than single handed weapons even if implemented with the two-weapon fighting style.
Not really. It depends on which weapon your looking at. Many of the one handed weapons do 1d8 damage. Most of the two handed weapons do 1d10 damage, and only the Great Sword and Great Axe can hit 12 damage.
2nd) The extra strength that goes into wielding a larger damage weapon two-handed should be abstractly thought of as the strength required to control the heavy weapon.
[In game physics example: F=M*A]
A lighter weapon is swung faster with less mass.
A heavy weapon is swung slower with more mass.
The heavy weapons already gets more damage, so why add even more with 1.5 x Strenght mod.
Point two is, in my opinion, flawed reasoning. A heavy weapon with the same force behind it will carry more kinetic energy then a light one. That can explain why the weapon has a better dice range. As for two hands being used to get 1.5 damage, that is simple too. With two hands, your applying more force to the weapon, therefore, more kinetic energy. This of course, leads to more damage.
3rd) The damage done by a finessing fighter is small in relation to that done by a high strength, two-handed weapon wielding fighter. A high DEX lower STR fighter alreafy has the disadvantage of lower STR mod and no increase in accuracy.
The reason that the damage is lower is because he is using lighter weapons. Weapon Finesse is meant to improve the fighting ability of weaker melee characters, not make a high Dex equal to a High Str character in damage output.
I see no reason for Dex fighter to inflict more damage then a Str fighter. This does not mean that the Dex fighter is inherently worse. After all, he does have a better Dex bonus to his AC.
4th) Getting rid of the rule would further simplify overall rule mechanics.
Getting rid of the 1.5 Str rule would simply remove one special case that applies to one aspect of play. The rest of the rules are still as complicated as they were before. If your looking to make the rules simpler, your better off taking a shot at Attacks of Opportunity, or the Generation of Monster CR's.
Anyway, why are you so convinced that the two handed use of weapons is so powerful? Has this become an issue in play, or have you simply ran the numbers through and find the results unsettling?
END COMMUNICATION