Don't get me wrong I would defiantly prefer it that way as it would be a very familiar and easy to use system but we have to remember square enix not only has to but wants to make money so let me play devils advocate for a bit.

Part of making money is the mog station and one of the items you can buy is the phial of fantasia. Lets just be honest, why would they remove an item that they can currently sell me to solve this problem just to give me an even better version for free. That's my reasoning behind multiple pre-set characters. Once you have made the max number of premade characters (lets say its 2-3), if you want to change after that then you will need a phial of fantasia. That way it doesn't become obsolete. The other solution would be a race glamour that would only be obtained by buying it on the mog-station.

Having said all that I would prefer your suggestion 100%.