How to duplicate values from grouped measures into additional virtual groups

Now that you have your Years table, you could use an expression like this for your population to get the most recent non-blank population

Population =
VAR __thisyear =
    SELECTEDVALUE ( Years[Year] )
VAR __summary =
    ADDCOLUMNS (
        FILTER ( ALL ( Years[Year] ), Years[Year] <= __thisyear ),
        "@Pop", CALCULATE ( SUM ( PlacePopulation[Population] ) )
    )
VAR __Top1 =
    TOPN ( 1, FILTER ( __summary, [@Pop] > 0 ), Years[Year] )
VAR __result =
    MAXX ( __Top1, [@Pop] )
RETURN
    __result

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat