No, it really isn't, that's just a crack conspiracy theory that took off because people no longer have the capacity to judge reality by any metrics besides cynicism and memeability.
There really are billions of actual people using the internet all the time.
All models are wrong, but some are useful. The value of a theory is how accurately it allows you to predict future observations.
If AI-driven recommendation systems and a trend towards centralizing all discourse on giant social media platforms produces a result indistinguishable from the Dead Internet Theory, then the theory is a useful way of thinking about and engaging with the internet.
I think it depends on the platform. Like Reddit for example, bot accounts are very easy to create (or you just buy used accounts). I wouldn't be surprised if on the most popular subreddits that half of the comments are bot generated.
