They should not shave their beards because the beards are iconic to the group. However, it is entirely up to them if they choose to o so. What a fan says hs no bearing on what a person does with their personal appearance. Basically if they want to shave the beard they can it has no effect on the quality of their music.
ZZ Top's beards are not only amazing but they are almost as iconic as their music. If they shaved their beards or if any member of ZZ Top shaved their beard they would no longer be ZZ Top anymore, and would instead be some washed up rocker has beens who are no longer memorable.
ZZ Top is one of the most recognizable bands of its day, partly because of the duo's beards. Would shaving their beards influence the band's music? Would it in any way impact the albums they produce? The answer, of course, is no. To argue ZZ Top should shave their beards is to ignore the importance of their music and simply judge them based on appearance.
The rock band ZZ Top is all about the iconic beards. If they shave off those beards, they are no longer ZZ Top. They just become another aging rock band. Fans link the beards inextricably with the music, so unless they plan to make a new start with a whole set of new music, they should leave the beards alone.