I would say, that based on my experience on growing in Poland (I was born in 1998), the people become richer and suddenly everybody now seems to be minding own business. So after you fill some economic needs, the will to pick on people decreases. But that will is surely dependent on how much you trust others are think similar (and won't rob you).
Anyway, the parents' commenter point was > near perfect "ethnical cohesion", doesn't stop crime or increase trust in any way
I can't see why should it be true. Perhaps it doesn't stop crime completely, but perhaps it actually increases trust. Based on the argument above we can't know.
Why? Obviously the majority of those who emigrated did not do so with the intention to commit crimes. However some people did. The question is whether they are under/over represented in the emigrant population.
Another aspect is that high emigration generally results in better conditions for those who remain due to higher demand for labor. Which might also decrease crime due to lower unemployment/higher wages.
> Perhaps it doesn't stop crime completely, but perhaps it actually increases trust
According to https://ourworldindata.org/trust Eastern European countries have lower interpersonal trust compared to most Western European countries and especially Scandinavia.
Of course it's also a question of attitude. And people in different cultures might interpret the question differently and/or be predisposed to answer it in a certain way even. Then again being from the region myself I can't really say that people here trust others more than in WE, ussually it's the opposite especially outside of certain social groups.
The degree of ethnic homogeneity did not increase during that time, if anything it slightly decreased (or significantly in some major cities).
One possible explanation is that people willing to engage in low level crime simply moved to richer western countries because well they were (and still are) richer... e.g. in Norway Lithuanians are the second largest group of people who are imprisoned (after local Norwegians). The situation is similar in some other Western European countries. I'm not an expert but if I wanted to rob/steal from people and businesses I'd probably do that in Norway, Germany, or Switzerland rather than Romania or Lithuania. The risk versus reward ratio seems much better there. Also there prisons are way nicer (especially in Norway).