One thing I always think about when it comes to HTTYD is the Light Fury's existence and the possibilities that could stem from it. Don't get me wrong, I don't like the Light Fury and the sexist way it was created, but the fact that it's considered a sub-species of Night Fury is in a way exciting, because it suggests that there could be other sub-species of dragons. Whether that be other sub-species of the Night Fury or sub-species of different dragon, its exciting all the same!
We do have some other examples of this in the HTTYD universe, like the Death Song and the Slithersong, and also the Snifflehunch and the Sneezlehunch (These are just 2 examples- I can't think of any more off the top of my head). It makes me wonder what else could have been out there that was never discovered, or at least never discovered by Berk. Even if the Light Fury itself has been a little problematic, given the fact that it was made purely for a mate for Toothless and the design is very sexist, it has sparked some creativity and imagination within the fandom which I love. I love seeing everyone's OC's they've made, and all of the unique dragon species they've created, it's so fun!
One other thing I've always been adamant about, is that I don't think the Light Fury sub-species is "endangered" or "rare". As we've clearly seen in The Hidden World, the Light Furies are very elusive and extremely good at staying out of sight. And as everyone saw in the scene where Toothless went to the Hidden World as the Alpha, there were a couple Light Furies in the background, which suggests that there is more of them out there. And if Light Furies are that elusive and are also closely related to Night Furies, maybe there are also a lot more Night Furies out there than originally thought and they're just hard to find since they're so good at staying hidden. Another thing we saw in that movie (and HTTYD 2) was that Toothless had abilities that he never knew about, which is assumed to be because he was probably orphaned at a young age and instead was taken in and tamed by Hiccup, so he never learned how to be a Night Fury properly. So maybe the other Night Furies, who grew up in the wild with other Night Furies and dragons, all knew how to conceal themselves better and stay out of sight or away from people. I mean, the best way to learn how to be a dragon is to be around other dragons like you, and Toothless never had that, so he never learnt how to be as elusive as other Furies until the Light Fury started teaching him.
And yeah, Grimmel was known to hunt Night Furies and to "drive them to near extinction" but do we really think that one man drove an entire species to extinction? Maybe he believes so, but he also didn't know about Toothless (who would probably have been well-known by that time, especially going on so many raids with Hiccup), so maybe he missed some. I just find it hard to believe he hunted a very powerful, intelligent, and elusive species to near extinction. And then there's the possibilities that the Night Furies could just live somewhere that's yet to be discovered, which is why they're so rarely seen.
So whether the Night Fury population is really low, or they're just good at staying hidden, I don't know. But I do think that the Light Furies are common, or at least not endangered, and they're just hard to find.