The first is that even if every graduate was hired tomorrow, it still wouldn't be enough to outpace the number of older workers leaving the workforce. The Social Security worker-to-retiree ratio was about 5:1 in 1960 and is about 2.7:1 now, and still dropping.

The second is that most new college grads aren't filling the jobs that need filling. The most acute shortages are in fields like agriculture, construction, home health/elder care, meat processing, and hospitality. Unless new grads are going to start doing farm work or taking care of the elderly, there still aren't enough American-born workers to meet the needs of the labor market.

So basically, immigration solves a different problem than the one you're referring to. Yours is a big one too but it's a separate issue.