Looking at FN's list of discontinued products, the FNS seems to have fallen by the wayside. Was this not a popular gun or was it left in the dust by other designs such as the 509?

https://fnamerica.com/discontinued-products/

It was not a popular gun to begin with and has now been supplanted by the 509 series of pistol as you stated in the OP. The FNS had a rather lengthy list of issues. Also, it didn't really catch on in the LE market. There were some high profile agencies that issued them but many reported issues. Like Arizona DPS:
https://www.azmirror.com/2019/...ued-to-all-troopers/

Yikes. Wonder if they ever completely addressed that issue?

FN issued a service bulletin for the series, to replace the striker. I have an FNS Compact, but elected not to send it in (range piece only to date). Pistols made after 02/2017 should be GTG, but indeed the 509 series is taking their place.

https://fnamerica.com/customer...ns-service-bulletin/

You answered your own question. The FNS series failed to develop much of a following, and its days were numbered when the 509 was introduced.

Is there any niche the FNS fills that the 509 doesn't? I suppose ppl who want a FN in 40S&W will be disappointed, but then FNH probably decided that market was no longer worth catering to.

FN's decisions are generally hard to understand. They released their 503 this year, which would have been a nifty pistol... in 2013.

Got my FNS 9 back in 2018 for $359 new. It has been flawless for me thus far after a few hundred rounds. The 509 series is definitely better all around, but I still like the FNS for what it is.

Honestly the handwriting was on the proverbial wall. The FNS never could get off the ground from a sales volume standpoint, certainly not helped along given the 'soft' polymer issues that the FNX had, a circumstance that FN really never could get convince buyers that they fixed the polymer heat durability problem (find a new FNX-45 for sale, empty the magwell of its mag, and give the grip a slight but firm squeeze if you don't believe me). That debacle dogged the FNS line, even though anyone comparing the FNS and FNX side-by-side could instantly feel and tell the difference in their poly frames.

The FNS was never anything close to being a strong seller for us, but it was a gun that seemed to develop a cultish following. I thoroughly enjoy shooting my FNS9, and even thought about picking up the longslide version as well. But then the next debacle happened with the defective striker design with early guns, and that became the latest killjoy for the model. Given that FN spent the money to come up with the 509, it's no surprise that they had to drop the FNS which ironically had been the 509's nearest sales competitor, at least for us. In all honesty the follow-up 509 prior to the panic rush (sorry Para; I have to put in context) hadn't exactly been a sales demon either. It, like that other sales-slug Beretta APX, didn't make the Army's shortlist. The paring down process apparently wasn't all that close, considering how quickly those and the M&P were dropped out of contention. That kind of info wasn't lost on folks who were following the proceedings closely. P320s and Gen5 Glock sales roasted the anemic 509, and still would today if it weren't for the acute shortage of available product we find ourselves with these days. That's one positive; we got to rid ourselves of the dogs that otherwise never could sell on its own merits (assuming of course that it had any to begin with).