If you stop using subscription and decide to switch back to Lightroom version 5, then you would not be able to access Lightroom CC catalog in Lightroom 5 as downgrade of catalog is not possible.
However, you can export all images prior to stopping subscription so that you can import them in Lightroom 5 and continue editing on them.
... or you can switch from CC subscription to a stand-alone license for LR 6. This lets you work without interruption and only needs you to change the subscription model of your installation.
my operating system is currently too old for lr 6, so i could update it, however the newest system is pretty buggy, so i may wait until sept for os sierra.
what i am wondering:
if i get lr via cc, and then one day want to stop subscribing, will i be able to switch seemlessly to using lr5 again?
If your operating system doesn't support LR 6, then it also doesn't support LR CC 2015, and when you purchase the Creative Cloud subscription, Lightroom 5.7.1 will be installed on your computer. You will not get LR CC 2015 which can support your camera. You will not be able to open the raw files from a Fuji X Pro-2 with Lightroom 5.7.1. So, you could switch back to LR 5 seamlessly, but there seems to be no point to move to the Creative CLoud if your concern is using raws from a Fuji X Pro-2.
Even with the existing operating system, you could use the free Adobe DNG Converter to convert your RAWs to DNG, which can be imported into LR 5.
If you upgrade your operating system, then the advice given by McLion and Chetna describe what will happen.