I’m not aware of this being a known issue but unfortunately…even if it’s not, no electronic device is guaranteed to not suffer catastrophic failure such as motherboard failures beyond the manufacturers warranty period. This doesn’t mean it’s not quality though. Unfortunately things like this can indeed happen. Since there is no guaranteed timeframe that an Apple Watch can be expected to work with no issues (beyond the warranty period and of course a little extra protection if you purchase AppleCare which doesn’t really make it guaranteed to work but gives you a little financial relief if an issue occurs while it’s covered) it’s hard to say if this is an indicator of defect unless it was happening to a abnormal high number of watches using the same motherboard.
As far as you being told that it couldn’t be repaired…was this via Apple? It’s possible they no longer have the parts to repair it or something like that. Were you offered an out of warranty replacement (at the price required for an out of warranty replacement)? Or just told you need to buy a new watch? I just upgraded to the 9 series but my 6 was still working well as of today with no issues other than the battery starting to show it’s age a bit more so unfortunately I can’t say that I experienced a failure like you. You could always contact Apple technical support to get more clarity on why repairing isn’t an option. Otherwise, unless Apple determines there is a significant motherboard failure issue (it would need to be affecting a significant number of watches out there and would likely need to be an issue that occurs in a reasonable amount of time within the normal warranty period or relatively close to it versus an open ended timeframe) you may not have any options other than replacing it.