I don't know if tunesmith has completely formulated his thesis yet,
but it is too simplistic and unrealistic
to expect singers to pin their 'voice' and identity
to a single fixed place and moment in time...

Everybody evolves and changes throughout life, and their varying circumstances...

Except for a minority of regionalistic/nationalist singers who are so stubbornly proud of their specific birth roots,
that they will cling doggedly to their set regional identity until even after they drop dead...

..and yes, there is a commmercial market they can aim at making a living from...

ahem.. "Wurzels"... [but... yes.. that's right even a Scot can be a succsessful profesional Wurzel..]