If we're including devices that almost nobody has access to, the most future proof device is not the one that keeps backwards compatibility, but the one that only has one version that is still produced, since initial compatibility is the only compatibility.

Any tricks and additional uses you develop for the abacus will likely long outlive any IBM if it's worthwhile. That doesn't mean it's particularly easy to develop something people will find worthwhile enough to propagate forward though. Backwards compatibility only matters as long as you have something that ca take advantage of it and a need to do so.