Economist: If the economy grows stronger, employment will increase, and hence more parents will need to find day care for their young children.
Conclusion
A stronger economy will probably make it much harder to find day care.
Evidence
In a stronger economy, many day-care workers will quit to take better-paying jobs in other fields.
Evaluate
Since this argument is based on a Prediction, which is an implicit comparison between our Current World and this Hypothetical World, we want to think about what sort of similarities and differences there could be between these two worlds.
In the Hypothetical World of stronger economy, the author thinks that one change (many day-care workers quit to take better-paying jobs in other fields) will lead to another change (much harder to find day care).
We need to consider what else could change in this hypothetical world that could relate to how hard it is to find day care.
The author is assuming the balance between Supply of day care workers and Demand for day care workers will be tilted.
He states that the Demand for day care will go up in a stronger economy, as employment increases, and he assumes the overall Supply of day care will go down, since existing day care workers will leave the field.
But what if people who weren't in day care before are tempted to join the field of day care?
Maybe someone is working at fast food for $10 / hr and then realizes they could do daycare for $15 / hr.
As the current $15 / hr day care worker goes off to chase $20 / hr jobs in this stronger economy, there could be other workers or currently unemployed people who are eager to fill these open day care spots.
Goal
Since the big objection to the author's argument is, , the argument needs to assume that .