After a few weeks I finally have the time to get around to this. Pardon my late response, mods, as I know grave digging is bad. So, at long last I can add my 2 cents to the pile. Here are what I think are the major factors for the decline:
Contrary to what some others say Reddit is far, FAR from a replacement for forums. Discord is much closer to a forum, and has seen an incredible rise in popularity. Being able to talk on the spot, have pins, upload images on the spot, have pins, etc, are huge boons. The younger folk of today aren't used to the slower pace and comparatively clunky nature of forums, and are better versed in the former.
-Users scared off
Recently-ish the forums nuked an insane amount of posts and by extension mods and resources pretty much without warning, as well as going through a potential permanent shutdown. Naturally, the former was and still is a major shock to everyone, an the loss of precious posts, memories, and resources that will not be seen again is NOT to be taken lightly. To this day I myself am still ABSOLUTELY LIVID that it went down and I don't doubt it could have been handled in a far superior fashion. This, as well as the fact that those who've not checked in on the status of the MC forum as not down, has likely caused many members and groups in these forums to jump ship for places such as PlanetMinecraft, Discord, CurseForge, and so on.
-Minecraft is an old game
Minecraft turned 10 this year. Let that sink in. A 13 year old then is 23 now. From effectively a child to an adult that in some areas is able to smoke, drink, drive, be on their own, etc. A decade is a long time and a lot can change in that span. Many kids such as myself have grown up, and as such have either found new interests, got bored, or have became busy; sometimes all three of those!
With regards to account creation, bots are a big problem and massively inflate those numbers. However, bots are not the only thing roaming this website. People are, and even if they don't plan on doing anything, people will create accounts to potentially post with. I imagine many will not post at all, whereas some may just leave some posts numbering in the single or double digits, then fade away into history, never to be contacted again. It's haunting and creepy if I am to be frank. Adding on to all of that, Minecraft isn't quite in its days where it is still being discovered. It is THE best-selling game of all time. Everyone knows about it, and by extension are going to already know quite a few things about it, along with simply being able to discuss with friends in person, over Discord, or other means of contact outside of forums.
Then we also have the vicious cycle of low activity in itself causing lower activity.