Contracts are fairly easy to breach if you have a good lawyer, and I imagine Valve does.
Do you have an example to back this up with? This sounds like wishful thinking. Money is the easiest way to break out of a contract. You pay the other person enough and he'll walk away happy. You don't and he'll take what you owe him.
When you're talking about hundreds of millions of dollars nothing is "fairly easy".
As mentioned, this is the way of the future for many companies. When they're big enough it makes sense to distribute themselves. They can charge less and profit more. Of course, they can't charge less yet, but the day will come. Publishers will still be needed to fund smaller games, but they'll do online distro, too.
This has little to do with the future, this is about Valve trying to cut Sierra out of the profits they agreed to share with them earlier.
As for the future of distrobution and it being better that way, none of that is relevant. Valve made and agreement and is apparently not going to live up to the terms. Considering that valve has Itellectual rights per that agreement I'd say they are treading on thin ice, if they are found in breach of that contract that gives the intellectual rights back to sierra, then they may lose control of half-life all together.