Hey, I'm a 24 year old native NYer. Also a Florida State Grad in Econ and Urban Planning.
Gentrification is the topic du jour in NYC, I have a few opinions on the long term implications.
I'm Italian full-blooded. Born on Williamsburg's North Side but raised in Bay Ridge. My parents divorced at 13 and my mom lived on SI so I grew up there. New Dorp HS Graduate.
Brooklyn has seen heavy gentrification in its northern portion and its continuing, however gentrification effects different groups in different ways.
Some of the areas seeing heavy gentrification like South Williamsburg and Crown Heights have humungous Hasidic Jewish populations, they aren't going anywhere. Unlike other NYC ethnic groups, Hasidic Jews have no other base in the USA. Yes there are smaller communities in Lakewood NJ and Kiryas Joel NY but those communities are largely satellites of NYC, all of the young men there come to Brooklyn for Yeshiva etc. The Jews will stand their ground, because they have nowhere else in America that is comparable to NYC, Brooklyn particularly. Most of them live in buildings either owned by other Jews that will only rent within their community, or they own their homes. The area by the Wburg bridge and Peter Luger's steakhouse is the same as always.
African Americans (that is black people descended from slaves who moved north in the 1900s) will decline in both size and influence. Unlike Jews, AAs have several large communities and many have familial ties to the South. For man AA's, places like Atlanta offer the opportunity to live in a prosperous suburban community whilst still being in the middle of a large community. I lived in the North Atlanta suburbs for a year after graduating college and of all the black people I met, most were from Brooklyn or Chicago's South Side.
Caribbean Americans (blacks from the West Indies and their children) will see a slight decrease but outside of Crown Heights and the area immediately surrounding Brooklyn College, the areas where they predominate are generally not going to be gentrified. East Flatbush and Canarsie have no interesting housing stock and are very far from transit. There will be a long term decrease in their city population because they will suburbanize (Valley Stream and Mount Vernon), and because new immigrants overwhelmingly choose South Florida over NYC for cheaper housing and better access to the Islands themselves.
Hispanics..NYC will get more Hispanics but the types of Hispanics are vastly different. Puerto Ricans are both suburbanizing, and moving PA and Orlando, however there will always be strongholds because of the importance of the city itself to PR culture. Many people from the island move here for 6 months to work, and live off the unemployment benefits in PR for then rest of the year, then repeat it. Basically we will have less Nuyoricans and more FOB types. Demographically, Dominicans will become the new Puerto Ricans and Mexicans (all from Puebla state it seems) will become the new Dominicans. Other groups like Colombians etc. have enclaves in Queens but they all in all tend to immigrate straight to Northern NJ or move their after a few years. Very few South or Central American communities are generational and it seems like NYC is just a stopover in the USA for them.
Russians and other former Soviet Union peoples still continue to immigrate here, but it seems that Russians themselves are leaving Brighton Beach (except for the coop buildings) and moving to Sheepshead Bay and Manhattan Beach, however BB Avenue is still completely Russian retail wise. Lots of 2nd gen Russians move to the NE Shore in Staten Island (by Father Capodanno between Midland and Old Town Rd.)
Italians have been pushed out to an extent by gentrifiers, or Asians in the case of Bensonhurst. Unlike many white immigrants to America, us Italians by and large have kept our culture for generations. I'm 4th generation, never been to Italy, and my home language is still Italian (Sicilian dialect) its the same for all 27 of my 1st cousins. We have carved out a stronghold on Staten Island that is unlikely to change, Staten Island below the expressway is stereotyped (somewhat truthfully..) as unwelcome to non-white people, additionally most of SI is too remote to be gentrified and if anything the area around then ferry will be "gentrified" by native SIers looking for a more urban experience. Also, very fee SI Italians rent, most homes are owner-occupied or rented to friends and family. More than any other group, Italians tend to marry other Italians and keep our ethnic identity, in Brooklyn there is still a large population of Italians in lower Bay Ridge (below 86th street and east to the ocean) and in Bensonhurst (technically Bath Beach but I never heard this term growing up. Everything from the N train to the Belt Pkwy and from Marlboro Projects to Fort Hamilton Pkwy was Bensonhurst) especially below Bath Avenue and in the buildings along the Belt on Shore Parkway. Bay Ridge won't get gentrified because it is far and has bad transport (the R sucks). Many single Islanders who work in the city move to Bay Ridge, which IMO is the last corner of "old school, working class NYC" especially past 86th street and east of 4th ave.
Asians are a group I don't know much about but it seems that they have steady growth. However most Chinese arrivals are rural Fujianese Mandarin speakers from the mainland as opposed to the original Cantonese Hong Kong folks who mostly immigrate to Canada or California where they still comprise a majority. Outside of small enclaves of Pakistani and Bangladeshi folks around Church Ave from McDonald ave to Ocean Pkwy, South Asians tend to be more inclined to NJ than the city.
The biggest growth group IMO are going to be "non ethnic" white (ie: English/German whites that predominate the USA outside of the Northeast) transplants who seem to endlessly move here in droves and for the most part seem to be staying (note the abundance of strollers in Park Slope and Prospect Heights).
Unbeknownst to many is that there is still a steady flow of (mostly illegal) Polish and Irish people. The Irish go to Woodlawn in the Bronx and Yonkers, and the Poles go to Maspeth/Glendale in Queens. However most new Irish go to the Boston area and Poles to Chicago, cities in which they predominate similar to Italians and Jews in NYC.
in the 2020 census, I suspect NYC will be more white, more Asian, less Black and while Hispanics will stay at similar levels, the makeup of Hispanics in the city will be much more similar to the rest of the country than to the NE where PRs still dominate.
Excuse typos, on my iPhone typing this.