I don't believe the level cap has anything to do with balance. We are in EA and the higher levels aren't ready yet. Heck, levels 1-4 aren't even finished yet. There's still lots of spells to add and classes/subclasses to look forward to.

I too would like to see level 5 added to the EA when it's ready, but I think we have a ways to go before that's something we should worry about.