Exactly. Would we still see this trend in the absence of WW2, the collapse of fascism and the discrediting of racism and hypernationalism in the West? Even if we did see a similar trend, I'm doubtful that it would be as forceful or all-pervasive.
I agree, without the war, even the most liberal regimes in Europe would probably maintain more race/ethnicity/religious based restrictions in place for a far longer period. The scale of human destruction in Europe that occurred in World War I and especially World War II made mass deportations/exclusionary policies on immigration unpalatable to the international community. Additionally, without World War II, you would not see supra-national organisations such as the UN, OECD, EEC/EU to condemn and impose sanctions/criticism upon countries who violate international norms.
I imagine that countries in Europe would make immigration agreements for temporary workers on a bilateral basis instead. For instance, you might see Germany make an agreement with say Hungary or Poland whereby workers wages are partially held and paid upon return to their home country and the end of their contract. This was already the case in Eastern Prussia prior to World War I where Polish workers were being recruited for the farms (from Congress Poland).