Giants only drew when they played Brooklyn. Here are the figures for 1956
http://www.baseballchronology.com/Baseball/Years/1956/Attendance.asp
Polo Grounds was in Harlem and it was a few blocks from the nearest subway station and night games were a problem. There was also the problem of 3 teams in the city and usually when the Giants played either the Yankees and Dodgers were home as well. In those days all 3 NYC teams televised every home and very few road ones.
https://www.google.com/maps/place/P...7c7960e3c9b737!8m2!3d40.8299488!4d-73.9374058
The Giants could not capitalize on winning the World Series in 1954 in attracting more fans. Owners looked at amazement with the crowds Milwaukee was drawing after the Braves moved from Boston and teams started moving.
Getting back to the Islanders I do believe New York can support another arena simply because of the huge number of people in the market but
Barclays has not been the gold mine I thought it would be. The only thing certain is the current owners are not Charles Wang which by itself is a huge plus.