Can I please request that any Subscription remains valid, until the user decides to disable it, so all/any posts to the subscribed thread are sent to all that are subscribed?
I could probably develop it for your own forum for my constant fee as a custom work (could be about 1-1,5 hours of work in total — this could be probably an add-on and non-core-destructive change, but it would be greedy to resources, as it would constantly check and update the subscriptions database), but I will not implement it in the default version, or at least not that straight way. Currently, I just don't see a method which would be quick and reliable, except for catching user's subscription link from an email message.
This all is done intentionally, as we already have negative experience on such subscriptions. Users should constantly confirm that they are still interested in this or that topic; MOST users check only for about 2-3 replies, then forget the topic, but still waste the server powers, because each email takes a huge resource. Also, there could be an old topic with a lot of subscriptions, which let's say getting a new reply in 3 years. Most of the user email addresses could be gone at that moment, but they *all* will receive a notification, which they would ever not need to receive.