The fandom is definitely well beyond its golden age, and has been slowly loosing momentum and relevance ever since. With g4 finished, along with the cultural spotlight no longer aimed at the fandom, as well as a lot of, at least, the younger bronies growing out of the show as they transition into adulthood we don't really have the momentum or attention that we once did. Even seeing this forum now vs when I first created my account it feels like a completely different place, both culturally and in relation to the general population size
I also remember when a lot of brony content creators, especially musicians and rappers started to veer away from the culture; as creating mlp content was not a great way to be taken seriously as an artist in the more general artistic sphere and a lot of them were aiming for careers in this area.
Could the fandom make a come back? Maybe. I think it has potential. But it would require a serious cultural shift. That either being something like new content that grabs us all as hard as the original show did (though like all second waves i think there would also need to be be some additional force that would sustain the community after the inital hype), or maybe a new perception of the fandom such as a post-ironic revival or the use of MLP concepts as a symbolic movement representing something else entirely. But I don't think, if the fandom ever comes back, that it will resemble the fandom in its original golden age very much if at all.