It's absolutely untrue that you can't sharpen them. You sharpen them with a file.
In addition to my Toro power mower and in the interests of environmental preservation and lack of noise pollution, lack of fuel odor, lack of fire hazard, I got a rotary, non-motorized push mower and I researched the subject of blade-sharpening both before and after I got the mower. For a small lawn they're ideal!
What do you think people in the olden days did? Throw out their mowers when the blades got dull? People then were more careful and economized better than people nowadays do in this throw-away, disposable products world.