> Importing immigrants en masse is likely to cause a lot of social instability and isn't a decision that should be taken lightly.

Look at it as an experiment. See what happens if you allow mass immigration. If you don't like the result, do what you'd do with any other policy - change it. ...Although I guess you'd be stuck with the immigrants you already imported. And they'd probably vote to continue immigration, as they have in the US.. in that sense, this is one policy choice that is largely irreversible.