1) In 2013, 37 critical updates were released for Windows 2003. As of July 14, 2015, no new updates will be released for Windows Server 2003 and Windows Small Business Server 2003.
2) Unsupported products are more likely to be attacked by malicious parties, which may increase the cyber security risk to your business.
3) Payment processors may not do business with you if your payments are going through an unsupported server. Your business may not pass a business audit if you do not transition from unsupported software
4) An average security breach costs an SMB $50,000. Running unsupported software and old hardware can be more expensive than upgrading to a modern technology platform
5) Improved performance, simplified management, and more affordable storage choices.
Courtesy of Microsoft Community Connections