While ideally output would have been measured objectively — widgets produced or something

A widget production in a factory or workshop will always scale with the time put into work. 5 days will always be more productive than 4 days. All other factors being equal.

You cant measure non-physical work in that fashion. There is no objective measure that is used to measure software development productivity, for example. Similar would be any other work that involves any measure of creativity or inspiration.

Therefore your entire approach to this topic falls short at that point of not having differentiated in between physically measurable productivity vs physically immeasurable productivity.