If a toon was allowed to have all classes, Then what would be the point of having so many character slots? ( Except for different Races). The game would also lose out on the purchases of mounts since you'd just need to buy one. There are also not enough quests to get 2 classes to 72, let alone 3-4 more. I was able to get my knight to level 72 without going to the new zone but i still wouldn't want anymore than 3 classes to a toon, it just becomes way to costly and honestly, it becomes cheaper to just reroll a toon rather than buying 203948203984203802439840239834 daily reset tickets.
The beauty of this, and every other serious class-expansion suggestion made so far, is that it is completely voluntary. You say you don't want to level up 3-4 classes? Great! Don't! Even if every class for every race was available for every toon, you could still just level up 2 classes and be perfectly fine.
You do bring up a valid point about mounts. I agree that mount sales could be hurt if more and more classes are added. However, the cost to gear up additional classes will far surpass the cost of a mount. A player who is willing to pick up and gear an additional class, but that isn't willing to gear up a second (or third or fourth) toon, will net more money for the Frog than losing out on one mount sale.