SOLAS regulations (safety of lives at sea) has sadly put an end to many ships. Taking a ship out of service for a complete overhaul can be very costly and time consuming. If they don't meet current safety requirements they can lose their licence to continue in service, which ultimately leads to the shipping line selling their ships, converting them into floating hotels, or in most cases selling them for scrap. e.g. The Saga Ruby (formerly known as the Caronia) was recently sold for scrap despite having undergone a 10 million dollar overhaul and a 20 million dollar investment interest in her. She even met current SOLAS requirements, but in February this year she was found in Thailand being scrapped.
The SS France (later renamed Norway) was perhaps the last 'big liner' to be scrapped in 2008.