You may be proven right in the strangest of ways. Turns out quagga were only a subspecies of zebra, and not a separate species unto themselves.
Apparently partial quagga-like coloring crops up from time to time in wild zebra herds. There are attempts to breed such animals to produce a the full quagga coat. Some intermediate successes have already been released back to the wild.
Domesticated breeding may in fact have them "around" once more.