What makes a good convincing conspiracy theory is that has elements of truth. If you really get in to the groups of people who actually discuss and propagate this theory, they think this stuff is more coordinated, wide spread, and competent than it actually is.
I would be careful treating “dead internet theory” as plausible until you actually dig in to the stuff proponents of it believe, you’ll quickly realize they go beyond “there are a lot of bots on the internet, shitty boiler plate content, and astroturfing campaigns” to color revolution theory, NWO theory, and other Lyndon LaRouche esque stuff. Like it’s a deep fucking rabbit hole that you would not suspect from the surface level ideas that are often presented when talking about it.