Removal and recreation does work as I use it constantly and see for myself that it works

It all depends on how you script your games really.
For instance, I have props appearing by ent_create when player is in range of the trigger mechanism and they are removed when out of range (but not yet purged)and happily return when player goes back.
The same can be done for AI if you take care to either shut it all down and maintain its current pos on restart or write text files and compute its likely position on second intervals or something in order to remerge it with the scene.
A lot games display rather well I thought.