As Nigel says if the advertisers don’t see a decent return from their ads they will likely not renew. Most will have some form of analytics that allows them to see where visitors to their website come from be that ads here, Google searches, Google targeted (to individual) ads, etc. They also visit the site and if they see people are using blockers and there is not much traffic generated from here they will likely put their money elsewhere.
So think about it would you rather not see adverts or would you rather NOT SEE THE FORUM.
If we take ARC as an example, talking with Ketan his sofware gave him a very good indication of where visitors were coming from. He also found that his money was better spent providing myself and Neil with machines and accessories so that we could do the beginner series etc. His ad here became smaller and then went away. That was as good as a 4 or 5 page advert every month for about 3 years. But those that did not buy the mag did not get to see the articles.
Likewise with Amadeal, they got a lot of interest and sales from the review I did in the mag of their ELS equiped lathe, maybe they will think again when it comes time to pay for web adverts?
Also look at what some of the dating etc ads have been replaced with, 90% are for this and other Kelsey publications. That is not going to generate much revenue when the “gaps” are having to be filled with the site owners own products rather than just showing a blank box.