It's probably hard to come up with a better interface. A pocket calculator can be operated with very little cognitive load. It's easy to memorize because things don't move and it's easy to discover because ~every action is always visible.

It's also good enough. Most people don't need more than a pocket calculator. Sometimes I use WolframAlpha, very modern, but I type my query on a keyboard whose design predates the calculator (QUERTY in <1873 vs "10-key" calculator in 1902)