Foreign investors are NOT in Poland because the domestic market is so lucrative
No doubt, never said that's the main reason for mnc's to enter Poland. It's a 'make here' country, not a 'sell here.' The large companies that come to Poland do so mainly to produce their goods, outsource a back office function like customer service i.e. Chase, Goldman Sachs, Google, etc. Those companies are going to remain regardless of what Poland does. The ones that enter the market to sell care even less whether Poland is in the EU or not. As long as people are buying their products they could care less whether Poland leaves the EU and say makes a union with Russia and Belarus.
That being said, foreign mnc's aren't going to just leave plants costing billions of dollars just because Poland leaves the EU. In fact, Poland continues to receive FDI. LG is in the process of building what will be the largest EV battery factory in Europe. Only leftists care about the politics of doing business. To corporations that's for the most part a minor consideration of doing business. That's why Chinese companies go to Africa. That's why the US/UK does tens of billions worth of deals with Saudi Arabia. That's why Israel has Iranian oil pipelines despite them being their mortal economy. Money talks, bullshit walks. Do they care that Muslims and Christians are fighting it out in Nigeria? Do they care that the African miners are paid pennies an hour in Congo? No. As long as the stock and EPS remains high, most corporations and their owners don't care.
What do you think would happen if Poland leaves the EU and the most important advantage is gone
Not really. If anything Poland would have more fiscal freedom and could offer better tax incentives to corporations. If Poland left the EU, which it won't anyway, the large educated workforce - with a huge chunk of the youth speak English,
Polish politicians seem to overestimate the economic and political power of the country within the EU and also on the world stage.
Again, not really. Poland is the largest economy in Eastern Europe (not counting Russia). It has a fairly large population compared to most EU countries. Also, it's economy is 6th largest in the EU and it's the breadbasket for much of Europe. Thus, by just those few factors Poland is far more important than most EU countries