I subscribed here for years but stopped when my gaming time became to dwindle and I could no longer keep up. In all the years I subscribed here I can tell you honestly that I never did it because of the benefits, I did it because I love this website and want to see it thrive. In fact I think most of the other fellow subs back then subbed for the same reason. Therein lies part of your problem. I've said it before here but I'll reiterate.
Almost all of the "old guard" members here have moved on due to life commitments changing and the newer generation of members here aren't the same type of users. That's not a knock on the current community or staff just a fact of how things change over time. The past members/subs of this site were here when the site was still standing up and supported through donations and subscriptions because it felt like that was needed to help keep the ship moving forward. Most of this current community didn't see those startup days and know the website for the great place it's been for a few years now. The best analogy I can make to this old model of subscriptions/donations here is twitch streamers. Most people subscribe/donate to twitch streamers, not for the paltry benefits, but in the hope that their money will keep the streamer online and able to continue providing content the viewer enjoys.
With all that in mind, I think it's obvious why subscriber membership has dwindled. The above mindset of the current community coupled with the fact that you can access virtually everything on site website with a free account leaves very little incentive to subscribe. I don't think adding a few subscriber only tourneys or a few more giveaways will change that either. The only thing I think will truly incentivize a large mass of people to subscribe to the site would fundamentally change the model the site was built on, pay walls.
If only subscribers could see road maps, post in forums or the front page, etc. there would be a real incentive for people to want to subscribe to the site. Obviously though this would require a lot of changes to the way the site is run as well as how content for the site is currently generated. I have plenty of ideas on that but won't go into that much detail here.
I know some of you may dismiss this post as me talking out of my butt and that's okay. I just wanted to offer a different view point to the staff that may have not yet been considered.