Holidays

Since refraining from meat the whole 9 days is a custom not mentioned in the gemara, isn't it logical that chicken not be included (like fish), since whatever simcha there is in a meal was intended to be from wine and beef (or perhaps lamb)? And therefore does one really have to be strict regarding this custom with chicken also? In addition, does this take into account that unlike in ancient times or even in the Middle Ages, today people eat beef much more often than on special occasions (and wine as well), so therefore it does not have the same important quality nowadays to make abstaining from meat as a minhag meaningful? Thanks.