In your hypothetical example world, Linux would be on equal playing field when it comes to video/hardware drivers. So it could compete as a serious gaming platform. Without vendor lock-in, hardware would have to be more interchangeable, so can select their own choice of hardware, which can be cheap or expensive or powerful or quality or pretty etc and fit it inside an aluminium casing if they like. They could get a matte screen and a nice keyboard ...
Without Apple's market power behind it, nobody would consider using iTunes over a normal mp3 player.
There is demand for pretty computers, powerful ones, durable & sturdy ones, et cetera. That won't change without Apple. Sure it won't offer the exact same things Apple does today, but on the other hand it will provide a whole bunch of things that Apple today doesn't or won't. On the whole it will be neutral or positive.