The answer is New apps will not work with the system.
Legacy System uses old technology and programs that should be necessary to run old files and software to newer versions of Operating Systems. Basically, Legacy Systems works with compatibility issues and fix it for new system software to still run old types of applications. Sometimes, businesses cannot afford to use new software and hardware in an instance. They would implement a temporary solution like Legacy systems to still use their old systems. Because Legacy system is design for old software, problems may arise and will cause new software not to run at all if you install them in a Legacy system computer or desktop.