So, would you like to have an instrument which is otherwise historically accurate baroque guitar, but has modern metal frets? I don't understand why. Non-equal temperament is one issue (especially when playing with other musicians), but fixed metal frets would also affect the tone of the instrument. Perhaps metal frets would wear out gut strings faster than gut frets? However, fixed metal frets did exist on plucked string instruments during baroque era. Chitarra battente is a baroque era guitar that does have metal strings and fixed metal frets.