I doubt the reasons for these periods of protectionism were economical. Most of these countries had (and still have) companies which are very closely intertwin

I doubt the reasons for these periods of protectionism were economical. Most of these countries had (and still have) companies which are very closely intertwined with the government. Protectionism ensured that these companies would keep their market share and that the government and its sponsors could keep their (economic) power. Once they could compete internationally these protections were reduced. However for the consumer an immediate opening of the borders would most likely have been better, with the difference that most of the economic decisions would then be made by foreigners and with less influence by the government.