Firstly, things do not work in a magic fashion where you ban imports this month and they achieve economies of scale the next month. You need to have consistency of policy and, regretfully, Asad Umar is learning on the job that you do not put one tax, then remove it, then put it back, then remove it again. Do NOT rattle the market.
Secondly, are the imports banned? Just reduced I think not banned completely.
Thirdly, whatever the benefit of having the extra chunk from import going to local manufacturers is offset by the rising price of USD and with it, the rising prices of cars. More expensive cars will mean lesser sales and lesser economic sense for mass production.
Finally, the point of "banning" on imports was that it directly results in transfer of USD from Pak to abroad which increases our import bills and contributes to our balance of payment crisis. Hence, the you must show money in banks abroad and make payments from abroad to be able to import rule.