This is OT but: I just (literally just this morning) went to check out the Great Learning based on your mention of it here. Thank you for that!
However I think you misunderstand the point of the concluding line. It isn’t about whether or not people in general care about important things vs unimportant things. It’s saying that when things of great importance are cared for, then the things of small importance will also be cared for.
Note that the gist of the entire passage is that to change the world, one begins by changing oneself. It’s all about the macro and the micro, the root vs the fruits. The key connecting phrase in the last line is “at the same time.” In other words, he’s not commenting on what is or is not cared for. He’s commenting on the connection between the two things. Caring for the things of great importance and caring for the things of small importance happen together; they are never separate.
He’s saying that it’s impossible to (properly) take care of the little things if you’re neglecting the big ones.