There is no discussion; there's a simple and intellectually honest reading of the rules, and there's picking a fight with people in order to satisfy one's ego.
The rules don't have to prevent this. They do not have to account for all possible twisting or misunderstanding or the rules. They merely have to show what is the correct or intended way to implement a game feature. In the case of calculating a blast area, they provide clear visual examples in addition to a straightforward definition. The burden of proof is on those who assert that their absurd twisting of the rules is in fact correct; not that the rules fail to discount it.
4E encourages the use of battle grids to provide a visual reference for combat, and make determining things like line-of-sight, and areas of spell effects easy.
For sake of simplification, they make diagonal movement equivalent to straight movement on the grid. The PHB only refers to movement in this manner; it doesn't describe treating anything else on the battlegrid diagonally: not turning creatures or players 45º, not rotating spell areas, and certainly not arbitrarily deciding to count squares diagonally to determine an area.
A blast is, by PHB definition, a 3-square-by-3-square area.
Is the diagonal form on the grid originally presented a 3x3 area?
1a A 3 square x 3 square area consists of 3 columns and 3 rows
1b By simple math, an area constituting 3 columns and 3 rows has 9 constituent squares. Each row consists of 3 squares, and there are 3 rows, per the definition.
1c From the figure, there is an checkerboard pattern of squares that appears to constitute 3 diagonal lines by 3 diagonal lines,
with an intervening four squares between them.
1d The figure encompasses more squares than allowed for per the definition.
1e The figure is not a 3 square by 3 square area.
Code:
. . X . .
. X o X .
X o X o X
. X o X .
. . X . .
o = intervening square.
2a A 3 square x 3 square area consists of 3 columns and 3 rows
2b The distance from one corner to the opposite corner must consist of 3 squares, one for each row and column. Any additional intervening squares would not be consistent with 2a.
2c The figure presented has
five squares from "corner" to "corner".
2d The figure is not a 3 square by 3 square area.
Code:
. . 1 . .
. X 2 X .
1 2 3 4 5
. X 4 X .
. . 5 . .
From 1 & 2, the figure is not a 3 square by 3 square area.
From 2, this is in fact a 5 square by 5 square area, with 3 squares removed from each corner. That is not consistent with the PHB definition.
WotC provided a simplified method to deal with diagonal movement on the battlegrid, to help make the game more accessible to newcomers as part of their new design philosophy. Some may agree with this, some may not. The effects of this philosophy will be seen in years to come as the D&D community either thrives or dwindles.
One of the consequences of this particular choice is some minor mathematical incongruities, that largely pose no problem for people
who just want to play the game and have fun. One is that it takes just as many steps to take a wide diagonal detour, as to move in a straight line on the grid. Another consequence is that this fact can be abused by those who would seek to bend the rules in ways justified nowhere in the books. Since the books are written for "players, not lawyers", WotC had to trust that its customers have some modest amount of common sense, and so will apply a little thinking in applying the rules and considering what's is or is not consistent or intended.
People who argue "the rules say nothing against it so it's legal" regarding absurd misreadings of the rules do a disservice to the player community and help ruin this game for others.