What you are saying is correct: these Forums are completely outdated. There have been a few threads (
Forum Overhaul
, for example) where members of the community have come together to share their ideas on what they think should be done to the Forums to revive them. We have even received a response from Mod Lyon, who is a member of Jagex's Web Team. You should take a look at his response there. Basically, their time and dedication is being used to work on more important projects and until those are completed, they will not focus on updating the Forums.
As for subscribing to a thread, I simply post on it -- like I am currently doing on this thread of yours -- and it will automatically be added to my profile page. I will then be able to take a look at it whenever I wish to see if someone else has responded; however, I do agree that there should be a feature where one would be able to subscribe to a thread and an alert would be sent to your profile when someone has either responded to something you have specifically said or a reply to the thread itself.
In conclusion, we are all in this together. These Forums are undoubtedly archaic and if we wish to "compete", sort of speak, with Social Media platforms, we'd first need an overhaul to attract old and new users and be able to enjoy the features that are already out there. It's a matter of time and dedication, as I have previously stated.