Borders are defined based on things like rivers, mountain tops, and physical markers. Which inherently move with the plates, so plate movements are effectively irrelevant.
As to something moving out to sea, the land everyone cares about is floating due to lower density, if it moves a mile in ~15,000 years it’s not going to noticeably change in anyone’s lifetime. And chances are the border will have changed several times before that happens.
Rivers don't move with the plates, well, they do, but they also move in addition. I'm not an expert but you can see the California-Arizona border along the Colorado river doesn't line up with the current course in a few places:
If a river straddles a flat line you might get a kink, but that’s normal in rivers. Generally they are surprising resilient, consider the rather inaccurately name new river basically ignore a 480 million year old mountain range that’s in it’s path. https://en.wikipedia.org/wiki/New_River_(Kanawha_River_tribu...
I'm really holding on borders becoming irrelevant in a time far shorter than 15000 years. Humanity must be united into a single entity and explore the stars together.
As to something moving out to sea, the land everyone cares about is floating due to lower density, if it moves a mile in ~15,000 years it’s not going to noticeably change in anyone’s lifetime. And chances are the border will have changed several times before that happens.