It only becomes "impossible" if you start adding machines and OS's into the mix that the app will no longer run on.
Vista will still run VB6 apps. My guess is that 7 will continue to do so as well.. and if not, there is always virtualization.
Any type of hardware / os upgrades that your company may be planning needs to take your existing LOB applications into consideration. This is no different than taking your current version of Office or your email client into consideration.
PC's don't really have an expiration date. Even if you are stuck with XP you can get hardware that works with it and will continue to do so for quite some time. If you buy prebuilt machines, you may need to simply downgrade the installed OS. Which isn't that big a deal.
That said, you probably have about 3 more years before things become difficult, and another 1 or 2 after that before people no longer want to work in your IT department because of how ancient everything is.