Up until now our growth as wizards has been upwards: we've leveld up, and as we have leveled we have learned new spells.
But there isno reason why we can't grow sideways. Now that we've reached the highest level we cant level up anymore, unless KI raises the cap, but eventually we will stay at a certain level, i dont think that we will level up to infinity. So KI can create new spells, and give us quests in which we can learn them.
Can you think of any other way in which a wizard can "grow" without leveling up?
Hmm I can see your point, but there is SO much more that they could do with Wizard 101. Perhaps they could release something down the line that allows for people to create their own missions with NPCs and everything for others to play, that certainly would add some everlasting replay value to the game.
Also the game like you said will eventually plateau and reach a point where it will stay for good. This could mean polishing what they already have and working out the flaws that sometimes show up. The game could be balanced and rebalanced again to make sure all the schools have a chance in the PVP arena and even work to expand that as well.
There are just so many options for them to make and only time will tell which ones they will do.