Well, on the other hand, France has access to young talents that weren't born in France and develop them to be French players, so the investment is always more positive than negative.
I understand your point about the 2nd generations, but let me tell you from my knowledge of Portuguese immigrants (which I know pretty well and for sure its specific), that 90% of the 2nd generation in France does not have any kind of bonds with Portugal or are interested in playing for Portugal... unless they don't have enough quality to play for France and are looking for a career chance (and in that case, we should have attention to alert on those cases).
I think you have pertinent points, and if Da Costa case was the generality of the cases, it should be reflected. But its not. And for his education, as long as their parents have been responsible and honest citizens, paying taxes and doing the work that the locals didn't want to do, I think it was a fair deal. But as I said, the majority loose the main bonds with Portugal, and feel French. And if they don't feel more French, thats mainly an issue of difficulty of France integrate the 2nd generations, Mr. Le Pen, and so on. They know their future is France, and the roots and some holidays were in Portugal, nothing more.
Portugal is now living changin times in immigration. We used to have, until the 80's, long term immigration (South Africa, Canada, USA, Switzerland, Luxemburg, Venezuela... and essencially France). Those people would work an entire life, half to stay ever in that country, half to return to Portugal when retired. Their kids were raised as Portuguese, but the vast majority doesn't feel Portuguese. Nowadays we have essencially short term immigrants (6 months-2 years) and expats (capital knowledge).
But the great change is... having immigrants in Portugal! It all started with the Africans ex-colonies in the 80's, and nowadays its essencially Eastern Europeans and Brazillians. The first ones for short periods (4 years, 6 years). the seconds for larger periods, but wanting to return to Brazil after 10-15 years, or earning a pension. And we need them, indeed.
In football terms, that means that Sporting, for example, as a Romanian kid who is 13 years old and is a great talent, and we also had a Brazillian kid (Alison) with 17 years old, also looking like a bright prospect.
Personally, I would feel honoured if they choose Portugal as their country. If Alison chooses Brazil... I feel it is normal also. I would still give them the choice to know in their hearts what is better for them.
The former ex-colonies sons of immigrants, many of them play for Portugal (e.g. Miguel), a few chose to play for Angola (even white players like Figueiredo). Its not that common, as its not going to help them in their career... but if they do it, its because they really feel more attached to their roots than to Portugal, and I think its normal. Probably we failed in those cases to make them feel like Portuguese.
For me, its always about individual choice, as long its based in real individual affinity, and not career goals.