Why should productivity magically be able to grow without bound? Drawing analogy to money (which, as a fungible, mostly incorporeal quantity, can reasonably change by X% steadily) is completely disingenuous.

Productivity is not fungible. The last 2% gain looks insanely different than the first 2% gain.

A better analogy might be weightlifting or running speed. And talk about high performers there to understand how huge a 2% (let alone 5%!) gain would be.