No one will complain if posts in those Mega threads are not counted for both post count and activity. It can be done by the forum (that I doubt the forum will not change its algorithm) or by managers of campaigns.
In some threads, disable post counts for paid-campaigns in Mega threads will reduce total posts inside but in some special threads, like Wall Observer, there are still lots of posts made. People made posts in Wall Observer are truly WO's citizens and they love their 'WO house'. I know some of them made their posts in WO more than in other threads.
There is an interesting thing about spam mega threads is OPs usually abandon their threads.
I would like to know the point of view of posting in such threads.
Don't post when you don't have valuable ideas to contribute and discuss as the core purpose of the forum, presented there:
This forum exists to provide a platform for the free (but ordered) exchange of ideas. If you have an idea to express, then it is probably possible to do it here as long as you follow the rules.