Only you can't use it forever, or at least shouldn't...
At some point your $400 version of msoffice will reach end of support, at which point security bugs will stop being fixed.
You might still be able to use it on a standalone non networks host which doesn't have removable media, but it will become increasingly risky to use it on a device with any kind of network connectivity and especially one where you're exchanging documents with other people.
Aside from that, exchanging documents with others will become increasingly difficult as newer versions will make changes to the format, or introduce additional things which are not compatible with the old version.
You might also find that the old version will no longer install on newer operating systems, or may not run on newer hardware. You have no recourse in this case, unless you want to run it inside an emulator with a corresponding vintage OS.