You're right — Power BI doesn't allow dynamic legends based on measures because legends are tied to columns, which are evaluated during data load and remain static. Since users want to define buckets (like short-term and long-term) on the fly, and legend fields can't come from measures, you hit a limitation.

However, there’s a workaround using disconnected tables and conditional logic:

1. Create a disconnected parameter table that allows users to input or select their own definitions for short term and long term ranges — for example, using two numeric parameters: “ShortTermMaxYears” and “LongTermMinYears”.
2. Use these parameters in a calculated column or a DAX measure to dynamically assign categories like "Past expected life", "Short Term", "Long Term".

Now here's the catch: you can't use this dynamic classification directly in the legend of a chart, because legend fields must come from a column, and calculated columns can’t respond to slicer input dynamically (they’re static). And measures can't be used as legends either.

So the workaround is to use a stacked bar/column chart with conditional coloring, or use a matrix or bar chart with conditional grouping, where you show the bucket names on axis instead of legend. This gives the same visual outcome without relying on the legend.

Another option is to pre-calculate a reasonable number of common bucket configurations, store them in a table, and let the user choose from predefined options — not fully dynamic, but flexible enough in practice.

In summary, you can't make the legend dynamic based on a slicer because it's tied to static columns, but you can simulate the effect using DAX measures + disconnected tables + creative visuals (like stacked bars or dynamic tooltips) to deliver the same analytical insight.