You will have a pointer when you disable signatures, avatars for a year, only then you can really know, though there is no point doing that, because *we already know.
We can have a glimpse of what would the forum be like without paid-for signatures in the Serious Discussion/Ivory Tower where signatures are disabled. Not completely dead but way less active than the rest of the forum.
Then there's the Wall Observer mega-thread with even >100 posts a day, often memes and shitposts despite also having disabled signatures. So it's not like there's a clear division between paid-for posters and serious contributors.