Hay knives aren't obsolete in the slightest, but are no longer made. I use one every few days for cutting portions from round bales. If you think that a chainsaw works better then you haven't used one in good operating condition. They work fast and with little effort. The "Lightning" or "Weymouth pattern" hey knife was most emphatically not an English invention, but rather an American one--specifically that of a George F. Weymouth March 7, 1871, of Maine. They were first made by the Hiram Holt Mfg. Co. of East Wilton, ME. The English pattern hay knife looks rather different, with a single canted handle and a blade resembling an overgrown chef's knife of sorts.
Here's a scan from an 1893 illustrated David Wadsworth & Son (of Auburn, NY) that shows both a Weymouth pattern hay knife and a few sizes of English pattern hay knives.