Why is KID considered more sound than Pirc? I have an impression that at the high level, the Pirc defense is considered to be much more off-beat than the King's Indian defense; at least the latter has been a main weapon for many top players, starting from Bronstein and Geller and through Fischer and Kasparov to Nakamura, while the Pirc is mostly seen when Black is in a desperate must-win situation.
However, I struggle to discern the reasons for that. The main difference seems to be that in the Pirc, White plays Nc3 with a pawn on c2 rather than c4, but isn't it a concession on their part? Doesn't having a pawn of c4 just give White more control over the center?
The c4 square is a natural development square for White's light-squared bishop, which targets the f7-square. Having a pawn on c4 takes away that opportunity.
In semi-open defences against 1. e4 the f7-square is shielded by a pawn on e6. But this creates dark-square weaknesses in Black's position when they have a fianchettoed kingside bishop on g7. So Black has to be careful of liquidating pawns in the centre in the Pirc, so as not to expose the f7-square to the White bishop.
Liquidation of pawns in the centre is less of a problem in the King's Indian Defence, because White is then left with a pawn on c4 which hems in his light-squared bishop (denying it the c4-square). So White doesn't gain a natural advantage by liquidating the centre in the King's Indian, but it's a viable proposition in the Pirc.
These two key strategic reasons are why the Pirc is &quot;less sound&quot;, or more difficult to play, than the King's Indian Defence.