You're right in that once you've shipped your app, there's no way to completely prevent tampering; every form of DRM is simply a means of raising the bar higher and higher. The trick is to make the bar sufficiently high by structuring your application well and using high-quality licensing and obfuscation tools.
The reason it's news is that Microsoft has been plugging CAS as a means to tamper-proof applications targeting the .NET Framework. Up until recently (i.e. the release of the Reflexil plugin for Reflector) there was no easy way to perform step 2 - that is, remove the strong-name requirement from the .EXE.
Now there is, which means that companies that depend upon CAS for DRM need to change their approach. Note that companies depend upon this aspect of CAS for more than just licensing; e.g. some rely upon CAS to prevent tampering in the case of scientific instruments that handle legally and clinically sensitive data. It's important for those companies too to consider ways of raising the bar.
The only solution is to realize that once someone has a copy of your program, he can do whatever he wants with it, and choose a business model that's compatible with that reality.
That's akin to saying that you must realize that when you lend your car to someone, he can do whatever he wants with it, so car rental companies should just choose a business model that doesn't involve cars being returned in working order.
If you want to implement an open-source business model, then go right ahead & do so. Many companies do, and are successful. In fact as a strong proponent of free markets I'll be fascinated to see if either the open-source or proprietary model comes to dominate; my suspicion is that both models are viable and will wind up servicing different sectors of the market.
Some individuals and companies choose to follow the proprietary model, and stipulate restrictions on the use of their software (such as not giving away copies of it). Ideally people would abide by the terms of use they agreed to when licensing proprietary software; in practice many people don't, so there's a market for licensing and obfuscation tools.