Given that Nishiki these days is a label for low to low-end hybrids, "mtbs" and kid bikes you'd not want anything they could provide you with even if they did replace this under warranty, so do not feel too bad. As the original purchaser, you've had almost 35+ years of riding this bike, that's pretty good.
FWIW, that "sudden" failure probably started with a crack some time ago which slowly spread over many rides until it reached the point where one more bump caused it to continue around the tube in what looked like a sudden failure, but what was in reality probably years in the making. It's a good reminder that those of riding old bikes would be well advised to clean them well and inspect them closely at least once per year.