Hi
Without a crystal ball I cannot guess what problems you will face, but let me tell you a story about a client of ours....
Many years ago, they were happily running some PB6 apps on Windows 2000 having taken the same view of not upgrading them. They were in the middle of a £40M upgrade to their oracle database infrastructure and rolling out upgrades to windows too when they hit a problem with the way DWs were working in the proposed production environment.
The whole project was on hold when they called us for help. The costs to the project were enormous and they needed a solution right now! We upgraded them to the latest versions of PB and they have used our services ever since having realised the error of their way. The problem was caused by a windows update something that could have hit their production service at any time. Unless you freeze all the environment you cannot be sure the apps will work in the future. Freezing the environment is a major security risk as you will not receive any MS updates and subsequent security patches.
So to answer your question, the apps can stop working at any time unless you run the risk of a major security breach and do not update your operating environment. Either way the cost can be significant to resolve in a hurry where as a planned migration to the latest version is much cheaper and easier to manage.
Hope that helps.
David