The article says this:
Jump of £37bn in budget deficit by 2040 would force government to increase taxes, NIESR predicts
If you were prime minister, would you cut immigration even if it reduces GDP and requires tax rises, or would you keep current levels of immigration?
Fixed it for you.
Like the stock market, only the rich people who own almost everything are directly effected by the GDP growing OR shrinking.
The only way the rest of us are hurt by either is through their unnecessary reactions to their unfathomably vast dragon hoards not accumulating at the rate they expected.