Yeah, activity points not only increased because your activity points will decrease by 14 points every 116 years.
That's not what 'lose out' was supposed to mean.
Rather (as suchmoon pointed out), since every activity period is not exactly 14 days long, but a little bit longer (+ 6 minutes, 40 seconds), you just don't gain that 14 activity.
However, that would not lead to your activity decreasing, but leads to the start of the new activity period slowly shifting further back.
Ofc, there are reasons of activity decreasing, e.g. when you (or someone else) delete all your posts in one period.
Or for potential activity accounts (as pointed out above), when their post count (because of a deleted post) falls under their activity.
Besides there is a chance we might be using the new forum software in 116 years... not so sure about that one though.
Oh, dream on.