I remember there was a similar problem with the Korg M3 touchscreen. There were many defectious touchscreens, they were not responsive in some areas of the screen. For a certain time Korg couldn't provide the fitting spare part any more.
That was because they ran out of stock, and they had to wayt till Korg themselves could buy new ones with the touchscreen company..
Soemtimes people forget how companies that dont create all companies are very dependent on the manufactorers.. Thats also the reason they had to take the Oasys out of production and many other products, because the manufactorers of the parts just stopped producing and selling them..
Same probably goes for the pa 800 series mainboard.. If Korg runs out of stock, they cant help you, because they are not manufactored anymore by a 3rd party. I think electronics where never meant to have an eternall live, and with the current state of technollogy where often parts cant be repaired anymore, you must hope that there is still replacement parts when yours breaks down..
Legally you cant blame Korg for not having the part available anymore, and it might just as well have been any other company out there suffering this fate..