Thats the thing though, another fort with the exact same settings had goblin sieges for 19 straight years until I abandoned because my framerate was so awful. I had a magma pipe, chasm, and cave river in a 5x5 area with lots of cliffs - it was fine at first, but pathfinding once I had a decent population was horrible, and wildlife made it even worse.
My next fort was 3x3 desert area with no features except a single layer aquifer and very sparse trees on 1/3 of the map - it didn't even have any little murky pools. The fps was great (~500 at the start) and the time flew by, and I had goblins come on the 2nd 3rd and 4th winters and had to get my military going and fight them off. Then the 5th and beyond winters I had a strong military ready yet the goblins just didn't come...
The only thing I can see that might have somehow caused them to stop coming would be the death of the 3 legendary goblins, they all came on siege #3 and as soon as they died the rest of the siege fled, and next year nothing came. In my other larger fort I only ever had one legendary goblin come, and they still kept coming.
I wonder if theres a trigger to turn off invasions once enough damage is done to the invader. Maybe the sudden death of 3 of them triggered a permanent retreat? These three were super badass goblins with hundreds of kills and a big ol' pile of goblin/human/dwarf bone trinkets, so if such a feature exists it would probably be a big hit for it.