To me it seemed that while the NY GOP was in the minority in the 1990s, it was a decent-sized minority. But during the 2000s they went into a death spiral.
Looking at it from an outsider's perspective, there were three things that I noticed. One was Charles Schumer unseating three-term Sen. Al D'Amato in 1998, which I think was a big loss for the NY GOP. Second was Hillary Clinton's victory in 2000. The Clintons relocated to NY and set up their national Democratic machine there. Finally, George W. Bush ruined the GOP brand in the state.
I would generally agree with that. The GOP did have some strong pockets locally in the NYC suburbs even after the suburbs moved Democratic nationally. However, that then crumbled. The GOP showed signs of getting the local organization back together, but that has seemed to go by the wayside. Casing point Nassau County. While I haven't been able to be involved politically this year (due to my current work schedule), things don't exactly look good for the GOP here in November, especially considering NIFA just ripped Mangano and company a new @SShole and the Controller's report (a Republican who backs Mangano) wasn't much better.
In order for the GOP to gain back any statewide relevance they need to regain and actually sustain themselves locally first. And while they have regained it in some areas as I mentioned, sustaining it is another story.