Okay, so I’ve read at numerous places that the French defeats at Crécy, Poitiers and Agincourt actually contributed a lot to the centralization of the French state on account of most of the nobility dying in these battles, leading to a rise of royal power.
That's a bit exaggerated, slightly more exaggerated for Poitiers, definitely more for Crécy.
Not only the process of unification and bureaucratic centralisation was already ongoing, but it didn't really prevented french nobility and upper-middle nobility to remain a main social force, during the Wars of Religion, for instance..
Don't get me wrong, the important losses at these battles, especially Agincourt, effectively deprived France of an important part of its traditional social-military forces, but we're talking mostly small to middle nobility there, not a decimation of the aristocracy even if the losses of several great nobles including people with commanding charge effectively crippled the political capacity of French which were already suffering from a civil war.
Long story short, the impact was moral and political; rather than institutional; and unification of feudal states doesn't really goes trough decimation of its aristocracy : if it was the case Hungary and Poland would have became unified kingdoms after the Mongol invasions, which wasn't really the case.
Unification (rather than centralization) in France passed trough the establishment of a feudal hegemony by the Capetian kings (a constant policy since the XIth century, which really worked from the XIIth century onwards), a strong fiscal network (which allowed french kings to benefit from ressources either the aristocracy, either neighbours as England), and a ongoing professionalisation of the army since the XIVth century.
@Byzantion
The problem wasn't that gunpowder did created a military revolution (the XIIth knew something more radical and the aristocracy did relatively fine, even if it pushed the small nobility down and really quickened the decline of nobility as a social-military class), than it happened at a point where the said aristocracy either already lost before royal authority (Spain, France), or where they managed to form semi-bureaucratic states of their own (as most of German states).
With the XIIth and growing costs of supplying armies (levies and/or mercenaries), it became harder for the smaller nobility to really compete with the suzerains : in the late XVth, only Brittany and Burgundy could really think about competing with royal armies, and maybe Armagnac or Foix as well. Not because gunpowder had to leave the battlefield (a short look at late XVth ordinance proove the exact contrary), but only great lords that went to a similar build-up could really hope keep up the pace.