Even though you are able to install external pip packages use of external pip libraries in Python plugins
Though, it might be a pain for plugin maintainer to keep it working with Windows, Linux, Mac, (possibly Android) and any other potential platform that Krita will be shipped. I mean, it is a good workaround, however I am not sure it is a good way for paid plugins where the user will expect easy and flawless install. And 80% will have issues with installation, it is not good.
For that matter maybe it is a good idea to ship pykrita with most popular image processing frameworks such as: NumPy, Pillow, skimage, etc
I know that there are pros an cons in this situation, but still.
Another alternative would be to give plugin developers a mechanism to expand their plugin with packages. Maybe something like ‘required’ section. The developer puts a list of required packages and krita automatically installs them if they needed.