Almost no SP-focused title (like BL) keeps its playerbase longer than three months after launch.I have to disagree. Good games keep their numbers, and even increase them pretty soon after launch when the previously suspicious to the series also decide to buy in on the fun. The game you reference to later in your post is one of them (TF2).
If a title wants to maintain player engagement, it needs to drop regular DLCs, be an MMO or have a strong e-sports aspect.Total War Three Kingdoms -- 90% of the average player count gone in three months. Kingdom Come Deliverance -- 90% gone in three months. Some games manage better numbers through DLC releases, which confuses things.