We are powerful, spacious, blessed with natural resources...
Good. Let's keep it that way. One way we can do that is to keep an eye on Total Population.
Hell, with projected declining birth rates, we could afford to lose 50,000,000 to 100,000,000 through attrition (
natural die-off, the passing of my own Boomer generation, etc.) over the next 30-50 to 100 years, going back to 1960s or 1970s or 1980s levels, without getting radical about population-size control, and still be just fine; with more resources to go around per capita, and with our grandchildren or great-grandchildren being the better off for it.
No need to continue thinning-out our 'blessings' now that we're done growing coast-to-coast and top-to-bottom.
...and possessed of a culture that has and does draw and assimilate the best and the brightest legal immigrants from around the world...
Every sponge reaches a Saturation Point.
We are not obliged to continue soaking-up outsiders until we are saturated.
There's probably always going to be room for a few 'good ones' here-and-there, but it makes sense to consider strategic goals and resource-consumption over the next century, when considering any next-round and so-called Immigration Reform, to decide, as a People, whether we want to continue bringing-in newcomers at present levels, or whether we'll more likely be better-off, scaling-back on immigration for a few decades.
...America's success is the success of immigration...
True. Until we reached the point where we stopped defending our borders and let-in 12,000,000 to 20,000,000 invaders, and tolerated them, in the name of Immigration Friendliness.
...and we have an enormous capacity for more.
In the 60 years between 1950 and 2010... we went from a total population of 150,000,000 to 300,000,000... doubling our population within less than a single lifetime.
We are neither obliged nor, in all likelihood, would we be wise, to continue admitting newcomers at present high levels.
Just because we have done a thing (immigration, at high levels) in the past does not mean that we must or should continue to do a thing in the future. What was a good idea in the past may not be so great an idea, moving forward.
It's up to The People to decide, in the end, and we may be reaching a point in our history when we need to take a long, hard look at our present way of accommodating and thinking about immigration, and incorporate that into the National Conversation.
Immigration is not a sacred cow, to be left unchallenged and untouched.
It is a means to an end, subject to a thorough and periodic review, and the time appears to be upon us, to put it under the microscope again, and to talk about it, and to make some decisions.