His main argument is that after seeing the game in action blast spells, particularly AOEs, are underpowered in general and that fireball is closer to where they ought to be. That's not the stance they had when designing 5.0, but over time at least Mearls has come to that conclusion. Part of the reason they got it wrong is, I believe, that they hadn't accounted for how many hit points 5e monsters have, so they didn't really realize that you had to go down to like CR 1 monsters for a fireball to actually start clearing out monsters and thereby reduce the incoming damage from them.