Why would the concept of senior disappear in 8 years? I am a senior, and work with seniors that have been seniors for 20 years. In 8 years we will still be seniors.
I consider 8 years to be the real experience to be considered a senior dev.
If from now on, the amount of junior is drastically reduced, this will lead to a lack of senior in 8 years because the senior leaving should be the same proportion.
In a situation where they replace juniors with agents, yes, we'll still be senior, but just like people capable of setting a VHS recorder, our number will dwindle.