As it happens, very few colleges actually require a high school diploma in order to be admitted. They require certain courses (which will be listed on the college or state higher ed site). This has been the case for quite a long time. No one in my FOO has a high school diploma, and all of us have multiple graduate degrees, mostly terminal.
Another thing to keep in mind about a high school diploma is that there are some jobs that require them and they can be pretty inflexible about it. I know a guy who dropped out of high school at age 16 to attend college full time. He attended for three years, got all As in hard core science classes (and everything else), and when he decided he wanted to be a firefighter instead, since he didn't have a high school diploma or "equivalent" he was required to get a GED. Obviously, passing the test wasn't a problem for him, but there is a stigma about the GED in certain circles because to most people it signifies dropout.
Of course, I'm saying all of this and I have two children who don't/won't have "real" high school diplomas because we homeschool.
