If French society is anything like Danish society, migrants from certain regions will use more in Government services than they will contribute in taxes:

https://link.springer.com/article/10.1007/S00148-017-0636-1

"Immigrants from Western countries have a positive fiscal impact, while immigrants from non-Western countries have a large negative one"

People from certain non-western countries have bigger disadvantages in respect with the language, education, either lack of, or degrees not being recognized, and financial means.

This has nothing to do with those people's culture or willingness to work or something like that. If a German immigrant had the same disadvantages, he would struggle just as much.

Furthermore, the point is moot anyways, because the economical value of a citizen goes beyond simply tax-payments minus benefits-received. These people are also customers that local businesses depend on, and any money spent on benefits will mostly directly flow back into the local economy.

Another important point is that these disadvantages disappear over the generations.

Another VERY important fact is that immigrants actually save us from the massive costs of an increasingly 'grey' society (due to low birth rate).

I'm not going to put every reason here how immigrants provide benefits. But I hope it's clear that there's a little bit more involved with determining economic impact than just tax minus benefits.

I visited Marseille a few years ago and the place is run down by immigrants to the point of no return. Filthy, druggy, prostitution everywhere.

Marseille has been run down by drug trafficking and decades of neglect by the French state not immigrants. In a way it has to do with immigration In that the reason Marseille was so neglected and poor is because it houses a large part of the Algerian diaspora which came after France lost its last colonial war something the French government would rather forget than address.