As discussed in this question and the comments there, we know, based on Beitza 4b, that there is some halachically binding concept of minhag avoseinu b'yadeinu (the custom of our forbears is in our hands).
What are the parameters in halacha for applying this idea? Meaning, we know it is applied as far as keeping two days of yom tov outside of Israel, but is not applied in other situations (eg many people are no longer careful to avoid mayim megulim because of the absence of snakes). How do we decide when to apply minhag avoseinu or not?
Why should this be so? Meaning, why should we continue doing something l'halacha (not just as a custom) just because it was done in the past, if the factors behind the halacha have changed (eg. 2 days of yom tov in chutz l'aretz even with a fixed calendar)