Late '60s/Early '70s
The top-loader sort of died out in this time frame, because front-loading portables became a lot more available at lower prices. It was one thing to get a front-loader when KitchenAid was one of the few choices. But when Sears made front-loaders available at every price point from lowest to the Lady Kenmore, it was a whole 'nother matter.
Top-loaders had one huge disadvantage besides the bending necessary for loading/unloading - their tops could be used as work space, but nothing could be left on top of them for long, since you had to raise the top for loading/unloading. In spite of ads showing portables being rolled into a perfectly-appointed, brand-new dining room, the reality of portables was that they they were often bought for older kitchens, and they tended to take root in one spot in the kitchen, usually right beside the sink. The only portable features used were the sink connector and the cord reel; they were almost never rolled around (and when they were, it was by-God hard on floors; portables are heavy as lead). Usually, owners just extended the connector over to the faucet, plugged in the cord, filled the detergent cups, and let 'er rip.
Over time, a front-loader's top would come to house the toaster and electric can opener, plus a canister, or maybe the instant coffee jar and the jar of creamer. Yes, the thing was still portable, but it became so firmly rooted to one spot that it might as well have been built-in.