I'd say I have some similar experiences to what you describe, and share equally similar ideas from those who have already posted, but I used to be in the same pool of "TF2 is dead" groups. Here's my retrospective on the matter:
As the years pressed on, I eventually came to think TF2 was experiencing a gradual death. The constant stagnation of consistent updates adding new content into the game led me to assume that it was a clear indication that new games were winning the "battle" against TF2, alongside this idea that gamers were moving on to those games and leaving TF2 into the dust (which I believe was merely an influence of many of my friends moving onto other games and giving me a feeling of alienation). In addition, it probably did not help that I had begun to experience a pretty serious burnout from playing essentially the same game for multiple hours a day for roughly 4 years.
Thus with all these elements combined at once, I retired the game in order to experience newer titles. However, it was thanks to TF2's 10th anniversary and the subsequent Jungle Update that a wave of nostalgia rekindled a new flame for playing it, and through this gameplay I realized the facts; Many of the dedicated players were still there, the fanbase encircling it was still producing excellent content, and new players are still coming from time to time. The game was never dead, but my perception between all of the underlying conditions led me to believe this.
The main point is, no matter how slowly these updates for the game get churned out, TF2 will continue to have a burning torch, so long as there are players in the game, and so long as there is a fandom to keep it propped up. Sure, it's not as big as it perhaps was in the late 2000's and early 2010's, but all that really matters is if the concurrent players are having fun.