TV won't be Apple's undoing but it also isn't obvious that they can win big there.
It is a complicated business with a number of interlocking players at different levels and vested business interests. It is also a global business with rights packaged up at a very national level and sold with exclusive deals.
While the biggest MSO's in each country (cable, satellite and terrestrial operators depending on the market) have the scale to lock in key content/channels to exclusive deals it will be fairly hard to break into the market in a devastating way. At the moment they have the scale and the revenues to do exclusive deals with the content creators (that they don't already own) to keep a large proportion of their customers from leaving. They (not the content creators/owners) are the big bullies in the market and would be hard and expensive to topple.
An AppleTV (including screen) if it does happen will need to be a good enough content offer for most people or it will not be a massive success. If they still have to use their cable box it can't be a game changing user interface because you are looking at the STB interface most of the time.
It's not impossible for Apple but it also won't be easy, particularly to go global. It will cost money to get content which might break Apple's principles and set a bad precedents for them. They also can't go that expensive because while I'm sure the UI will be better Samsung/Sony and some of the others really aren't that bad and the cosmetic designs don't leave that much room for minimalist improvement.
The business case for Apple is far from clear for me at this point but it would be a way to take the war to Samsung that would cost Samsung far more than a billion dollars even for an Apple relative flop. The collateral damage might kill Sharp unless they supplied the panels and would be another knife in Sony/Panasonic/Toshiba that they could do without.