If it happens, the confidence and price of the coin will collapse. People holding the largest amount will suffer the most. Therefore it wont happen.
That has always been the assumption, but it just isn't always true.
Bitcoin Gold suffered a 51% attack in May and its price has not been affected.
ZenCash suffered a 51% attack last week and its price has not been affected.
Verge has suffered several attacks over the last couple months, and its price has not been affected.
Monacoin suffered a 51% attack last month and although the price dropped temporarily, it returned to normal.