You make some rather big assumptions here.
- That they could just reclaim all the land that is now free thanks to dead people is any easier to reclaim that any other land around
- That they ever stop raiding each other (its an easy way for them to get stuff also glory etc.)
- That there is going to be one tribe that comes out of is on top of the other instead of many small tribes that raid each other all the time.
- That the plague will burn itself out completly and won´t stay around like some did rl and just keeps killing.
- That they are able to just jump back form losing 50+% of their total pop.
- hello extremly high pop. growth while the pressure that keeps it at ~5% or less per 100 years is still there
- That they will be able to get more food per person now then they did before and are able to use that to fuel the pop. growth
It is here that being a stone age civ comes to bite them as they don´t have any of the tool, knowledge or infracture that would allow for a fast bounce back from ~50% lose of total pop.
This doesn´t include what happens when "outside" force start to settle into the lowlands.