From the Agile Manifesto:
Simplicity--the art of maximizing the amount of work not done--is essential.
The act of prioritization is a manifestation of choosing what to work on next, to maximize value. Prioritising a piece of work for delivery is exercising the option - to seek to extract value from the item.
Product Owners manage a portfolio of options on a backlog. User Stories Are Options, Not Futures because there is no guarantee that work on the backlog will ever get prioritized. However by doing detailed analysis on a user story before it's prioritized for delivery, the option is exercised early.
As Future work is an option then there's no guarantee that this work will ever get prioritised.
Because We can only be certain about now, these may have practical implications when considering future work, because Future work is an option
As Future work is an option and User Stories Are Options, Not Futures - how can the prime properties of options, namely their window of opportunity (Options expire), their value (Options have value) be applied to ordering a backlog?
In essence, Future work is an option, whose value has some quantity (within a cone of uncertainty), but the means to extract that value is unknown.
However, Future work is an option with no guarantee that it will be started.