People who are asking for "more levels" should really be asking for epic level content. Having epic level content in a game/campaign that starts at level 1 could be too much. Personally I find epic level content more of an epilogue to a very long campaign, perhaps a trilogy.
D&D is not an action RPG where 99 levels exist just for grinding, and where monster stats are scaled with you and the game never really changes. More levels isn't automatically better. Quite the opposite if they're not done right. I'd also like to take my time with every level. In EA the levels already rush by and you're level 4 before you know it or remember what 1-3 were like.
Epic level requests is something where the developer has to protect the players from themselves. Or at least know what they are getting into if they want 1-20 in a single game.
Given the way we start, I'd be INCREDIBLY surprised if we don't end up facing some "epic content" by the end of this game.
That aside, speaking in a general sense, level 14 would pretty much be my ideal "sweet spot" between being able to experience high level builds, abilities and spells and stopping right before the system starts "breaking down" because everything becomes WAY too op to make any sense.
