I wonder how much overhead these alerts are generating and whether that could over time negatively impact performance due to all these extra DB queries. For active posters there could easily be dozens if not hundreds of alerts, even thousands on a busy board that has some history. Multiply that by the number of active users. How well do alerts really scale performance wise?
I noticed that the majority of alerts were created by thread subscriptions and have made a suggestion to change the default behavior of subscriptions: Turn "Watch this thread" off by default, possibly adding a usercp setting