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Graveler wasn't originally depicted with the larger feet it has now. This is the original Sugimori art for it, and you can see the Red/Green and International Blue (Red/Blue US) sprites all don't show it having feet. Or if they do, which is arguable of this picture and the RG sprite, that they are extremely small and difficult to notice.
The Yellow sprite, which came out in September of 1998 in Japan, shows the feet clearly as we see them today. Graveler had already debuted in the anime a year earlier, in the "School of Hard Knocks" episode, where it also has these feet. So at some point after the original games, it's likely that the design was altered to add these feet or make them more prominent to make Graveler easier to animate. It's also possible that Graveler's 3D model for Pokemon Stadium (Japan, AKA Stadium "Zero", which preceded the English game we got named Pokemon Stadium) was created prior to this anime episode, and that's when the design was refined to include the feet. Although Graveler isn't playable in that game, its model exists inside the game files/code.
So I don't think it really was intended to lose limbs upon evolution. Rather, it was supposed to have four arm-like limbs and the bottom pair turns into dedicated legs upon becoming Golem. Just my opinion.