I bet cupped keys are another urban legend. I think they were cupped to force the user to hit the keys dead center. And when that was no longer necessary, they were replaced by flatter keycaps which are actual superior in terms of speed and comfort.
Actually, the history is the other way around.
There were flat keys for old mechanical typewriters which had a round metal edge to hold in the clear plastic disk covering the paper key legend.
When keys were made out of plastic, they tended to have a spherical curvature, although there were some exceptions in the manual typewriter days.
The old IBM electric typewriters had square keys like a Flexowriter - which indeed seems to have an IBM electric typewriter mechanism. These had spherical depressions. And pretty much all other typewriter and computer terminal keyboards followed suit.
When the IBM PC came out - or, rather, its predecessor, the Datamaster - IBM had the brilliant idea of using keys with cylindrical tops. This still helped to locate the fingers, but because a cylinder is a 'reducible surface', plastic stickers with key legends on them could be applied to the tops of the keys. The popularity of the IBM PC drove nearly everyone else to adopt this style of key in fairly short order to make their keyboards look "modern". The IBM PC also introduced putting the key legends on the left side of the key, and the capital letters at the top to show that the keys only make capital letters when they're shifted (or when Caps Lock is on, but let's not get into that).
Cupped keys with centered legends, with the letter keys having big letters on them that filled the key, had been the standard before the IBM PC, used by everybody - DEC, IBM, Unisys, Control Data, NCR... and suddenly, IBM introduced a new style of keyboard, and everyone wanted to look like IBM... once again.
