Growing up on long Island when was a little kid, we had 3 just in my hometown alone. Nowadays, I don't think there are more than 4 or 5 (at the very most) in all of Long Island....
No doubt Long Island has lost many kosher & kosher style delis, but it is not quite as dire as you imply.
Omitting Brooklyn & Queens, these are the ones that remain, many I've been to (in bold
, 3 locations, Greenvale, Carle Place, WoodburyLido Deli
, Lido BeachWoodro Kosher
Max's Appetizing, HewlettKensington Kosher
, Great NeckRegal Kosher
, PlainviewPastrami & Friends
Pastrami King, Merrick
Zan's Kosher, Lake Grove
King David in Lake Success, now a Shake Shack
Andel's in Roslyn Heights, was the best source for smoked fish east of the lower east side.
..and one in Plainview I can't remember the name of
..and another in Merrick on Merrick Rd now an Allstate office.
Here in Fredericksburg, the 2 closest options I have are in Fairfax and Richmond. Neither one is as good and the worst on the list above.