It's not so much that the game is dying, it's that Niantic has failed to provide any significant "end game" content. As a result, once players hit level 8, and maybe grind some ways towards 16, they get bored. There's nothing to engage them anymore. Some may take on a leadership role in their community for a bit, but many who do quickly learn that that leaves even less time to actually play the game. Then they realize that they may not actually miss it at all.
You know what the difference is between a level 8 and a level 16 player? For the average person, hundreds of hours of play time. That's about it. Because as far as gameplay goes, once you hit level 8, you've got it all. The extra XM almost doesn't even matter - it's not like it will actually help you defend portals.
As if the boring end-game wasn't problematic enough, mix in:
Cheaters - You just spent 2 hours walking around out in the cold, tearing down the enemy farm and rebuilding it under your color? Here comes a bot to undo all that in minutes!
Local Drama - Every city has some; whether it's just a few players who can't get along and have to have their fights on public comms, dedicated trolls out to harvest tears, hardcore fucktards crying because you "blocked them", or legitimately batshit crazy players who can't separate game from reality.
Hardcore Players - Players willing/able to put more time into the game than most of us put into our jobs. No matter how "smart" you play, if you can only get in a few hours a week, you will never be able to compete with the guy who spends 6+ hours a day hacking, attacking, building, linking, and fielding.
Old Guard Butthurt - I suppose this falls more under the previous two categories, but it's become so common as of late that it's worth pointing out. There's this hilarious attitude you'll see with players who started in the first ~year or so, where they think anything that allows new players to progress more quickly than the old players did in the early days is bad. Or really any changes that make gameplay easier for newbies, casual players, or the locally outnumbered faction.
It's easy to imagine why some people burn out on the game eventually.
But, the game itself is alive now more than ever - more and more people are starting to play every day, and though the old bittervets may be disappearing, I genuinely and truly believe that you'll have more fun playing with other new-ish players than you would the old guard.