That won't change the situation for developers and users much at all.
If Apple allowed games designed only for game pads, developers would still have to design games for the ATV remote or live in a tiny niche market. It just doesn't make sense to do that. Why not develop your game controller-based game for a system where all the players actually *have* a game controller?
The only way for game controller-based games to be viable on ATV is if Apple releases one, and it probably has to be included with the ATV from the start.
Apple knows this. Whether it's wise or not, they've decided *not* to compete with traditional console games. They want developers to create games designed for the ATV and its included controller, *not* port the same game from other systems that plays the same way. It does make sense to do something different, since they aren't going to compete directly with the established consoles with a much less powerful system.
Actually it can happen another way, and has in the past.
You can sometimes get a 3rd party item, that becomes so popular it ends up becoming pretty much a standard thing most people would get, but such things need large scale 3rd party support.
Scenario would be an excellent, well priced games controller that was stocked in many places, Dev's saw it was a great device, and started to support it. a few smaller devs saw these large devs supporting it and they also supported it.
With a little luck, this snowball starts to grow, and after a while, it just becomes "the norm" for anyone writing a game to also support this controller, and it can become almost an official / unofficial product that "you just buy" to go along with the main product.
A bit like the old Soundblaster cards, Voodoo 3D Cards, Joysticks etc of the PC world of the past.
You just need a collection of the BIG Boys to support it, the smaller boys also supporting it, and the snowball effect kicks in.
The trick is, the hardware (joystick in the case) has to really stand out as something that's wanted to be supported by the right people so this can start and gain the momentum in the 1st place.
Naturally if Apple make one then none of this "hoping it happens" would need to take place, and it would happen from day one.