The WC-1 doesn’t work with the System-8, so not really an issue. But to answer your questions, PLUG-OUT predated the Roland Cloud stuff, so doesn’t rely on it. Instead, when a PLUG-OUT compatible machine is connected via USB, you can press the PLUG-OUT button on certain plugins in order to copy them over.
So it’s the plugin that matters, not how you own it. Since the introduction of PLUG-OUT, a new way of owning these plugins has come along (“leasing” them from Roland Cloud), but that doesn’t change how PLUG-OUT works. If your Cloud membership expires, the PLUG-OUTs you installed on your synth are still there and working. If you connect your synth to USB, the PLUG-OUTs are still there. If you transfer presets back and forth, the PLUG-OUT works. It’s been a while, but I believe that updating firmware leaves the PLUG-OUTs untouched, too.
If you want to get back to the jup-8, jun-106, and JX-3P that shipped with it, you can do so via factory reset and don’t need to own the plugins do it. But even there, you’ve got options in case you don’t want to mess with some existing PLUG-OUTs:
