Is the current point buy system balanced? No, but if the highest value available is a 15 score, it doesnt really matter, and isnt really abusable.
The main thing is, D&D has evolved beyond the math of the original 3d6 score generation. Today the values are considered in their own right, according how they balance within the game engine.
Example after background modifications, the following resulting arrays feel about equally worthwhile depending on the character concept.
+2, +2, +2, +2, +2, +2
+3, +1, +0, +0, +0, +0
For pointbuy generally, moderate high scores should be cheaper, and high scores should be much more expensive.
This is why I prefer the array for character generation. I offer three arrays, each about equally desirable. Players choose whichever makes sense for their character.