Variety is part of the fun of having stories from different cultures. The nuance of the social structure can be important to Japanese stories; that's why honorifics are preserved in good translations. The flavor reinforces that the relationships function a bit differently. I'm missing out if the translator sanitizes the language for English.
Also, English uses honorifics in its own way. Dr., Mr., full name, last name, first name, shortened first name, nickname, guy, etc. English has a variety of ways to refer to people from formal to informal that serve similar functions to honorifics. However, since English is more casual about it, it's difficult to translate directly without losing meaning.
A good localizer has to decide which term to use based on context. For example, if Jones is a teacher, Jones-sensei is probably pretentious and could just be Mr. Jones. However, sensei can be used in other contexts where Mr. wouldn't be quite right. Senpai is another term that can't be translated. No one says "Upperclassman Bob", but if senpai is completely omitted, then the social structure between the two people isn't reinforced. Since senpai-kohai relationships actually impact how people behave in stories, it's informative to keep the terms.
I also think replacing "onii chan" with "dude" is probably a bad translation, but not always. No one says "onii chan", but people could use "dude". But the connotation is different, so that's why keeping the original "onii chan" indicates "this is a foreign work and we can't translate to a word you know without losing some meaning." Unless the character also happens to live on the beach in Hawaii, I wouldn't go with "dude". For that level of familiarity, it would be more common in English to simply have no identifier. So instead of "onii-chan, how was your day?" just "How was your day?" But again, it's possible to lose meaning.
As a reader, I prefer keeping honorifics so I'm informed, rather than have a translator decide I'm an English speaker who can't handle or isn't interested in subtleties.