That comes back to the idea that people have the right to live where they like.
People who oppose gentrification believe that the government (or the "community") should try to influence who lives where. While it's absolutely true that people impose externalities on one another, this doesn't give the government carte-blanche to determine how people live their lives, or who should live where.
In the case of SF, the reasons range from the plausible (people of differing income levels living together contributes to the overall wellbeing of the city) to the discriminatory (artists are more valuable to the city's social life than tech works) to the downright nasty (tech nerds are bratty privileged rich kids)
People who oppose gentrification believe that the government (or the "community") should try to influence who lives where. While it's absolutely true that people impose externalities on one another, this doesn't give the government carte-blanche to determine how people live their lives, or who should live where.
In the case of SF, the reasons range from the plausible (people of differing income levels living together contributes to the overall wellbeing of the city) to the discriminatory (artists are more valuable to the city's social life than tech works) to the downright nasty (tech nerds are bratty privileged rich kids)