Airlines are watching here, at least to some extent, so what I write is not secret.
They are testing the water. Basic economy vs. economy fare differential varies. Sometimes $20 each way, sometimes $30. Some airlines will let you pick seats from the last 4 rows then they will assign probably mostly middle seats after that.
Travelers will be asked to assess things like:
If I am elite, will I pay $20 more one way on a transcon than basic economy to get my pick of seats, which might include exit rows?
If I am not elite, will I pay $30 more one way than basic economy and get an average seat versus a rear middle?
Someday, it may be...will I pay $40 more one way than basic economy and not get any water, have to pay to use the lavatory, and are prohibited from standing up except to use the lav or deplane?