Breach airlines T&Cs at your own peril.
Some airlines, such as LX and LH, allow you to choose the hidden city option - but they will price it based on your actual journey, rather than your ticketed journey.
Senior courts in many jurisdictions, and the UK's office of fair trading, have upheld the airlines right to charge their fare for the journey actually taken.
Oh, and for the very very few passengers who have to change due to unforeseen circumstances, many airlines allow them to do so, provided they advise them in advance.