Earlier quoted context omitted.
We need more if we are to sustain our tax base; immigration only seems high because fertility is relatively low. While it's a lot in absolute terms, the US is only around 180th in terms of population density, well below average. http://ssab.gov/Documents/IMMIG_Issue_Brief_Final_Version_00... http://en.wikipedia.org/wiki/List_of_sovereign_states_and_de...
Or we could consider spending less.
No we couldn't. Entitlement spending (eg social security) is mandatory, by law. The architects of that system (and of Medicare) did not foresee the bay boomer generation and the sudden drop in fertility. US pensions are not especially generous, though Medicare costs could certainly be controlled. I urge you to read the SSAB report; this isn't something you can conserve your way out of, same way that you can't conserve your way out of rising carbon pollution.
Sure, Congress could pass a law to cut entitlements significantly. But good luck explaining to voters who paid payroll taxes all their working lives that they can't have what they paid for.