Why should we curtail immigration?

Via Isegoria:
Why should we curtail immigration? the Economist asks:

How many is too many? Well, the foreign-born population of America peaked around 1890 at about 15%. Looking around me, I see that almost no one seems to be speaking Czech, Italian, Polish or Yiddish, or even English with a crusty Irish brogue, so I presume they were all assimilated adequately.

Currently, America's foreign-born population is about 10%. This suggests that America could increase its immigration by 50% without destroying its prosperity machine. It's harder to gauge in European countries, which have no established tradition of absorbing massive immigration flows. But it seems likely that most countries could take more than they have. Not endless numbers. But enough to make a lot of lives better. Including all of us who love ethnic food.