Depends on what the definition of 'niche' is. Which is impossible because everyone has their own idea of what is niche.
Once multiple installments start breaking the 1 million (or even 2 million) barrier, like what happened with the more recent ones, I think it's safe to say it's no longer niche.
So is persona no longer niche cause P4G broke 1.7 mil and p5 broke 1 mil
I don't think so. Constant 1 mil is not enough. Constant 3 mil plus is more like it. Metroid is niche, probably one of the more popular niche franchises but still niche. It always breaks 1 mil usually breaks 2 but never breaks 3 mil.