You can opt out of the 90 day deletion if you ask Support.
But another thing to note about inactive, you should use ANY or group filters depending on what your goals are. If you are scoring a lot of behaviors, then NOT Score Was Changed could be good. If you prefer to target direct behaviors, choose the ones that make sense for you. Make sure each filter has the Date of Activity filter on there.
So you might want people who haven't (Clicked Link In Email AND Not Visited Web Page) OR (Not Opened Email, etc). Or you might want people who haven't done anything at all in 180 days, so then you'd use AND for all filters.
Most firms I've worked with try to remove inactives or wake the dead 90 and 180 after days of no activity.