Not missing a thing. That's just how it works in Pro Tools(and it is a popular request). Some things to remember:
1-If you really can't pay attention enough to avoid dropping on top of a plugin you want to keep, you might think about working the changes in a different order.
2-you can use the track next door(so to speak) as a place keeper(if its got plugins, you could also create a new blank track and then delete it later. Just make sure that track is the same "width"(as in mono, stereo, 5.1, etc)
3-you can leave empty slots anywhere you like(I almost always leave slot A open for Melodyne, just in case).
4-if you are putting reverb or delay plugins directly on audio tracks, consider putting them on separate AUX tracks and use sends to "feed" those effects. This is often a more efficient way to use those effects, and would be fewer plugins that you might need to move
