" I think going to a major research institution is at least as good at grooming graduate students as a LAC provided that a student actually takes advantage of the opportunities available at said research school."
I think this hits at the key: it is (as they say in Ireland) horses for courses. Some students are ready, willing and able to be the student who puts themselves forward enough to stand out from the crowd in a big research institution. Other (equally able) students may not be, and may bloom better in an LAC environment.
It is a sample size of 1, but my D thought she might be interested in physics, but her one course in high school was not enough for her to be 100% certain. She chose an LAC over a research institution. First year went well, and she applied (and got) a (paid) summer research job sponsored by the LAC the summer after first year. That settled it, and she is now a physics major (and lab assistant), and for summer after 2nd year she has an REU at a big-name research institution (that she would possibly apply to for a PhD). In the meantime, she is loving life in an LAC- taking a range of courses, her ECs in her small community, etc.
The point is that she has grown into herself in the LAC environment and the opportunities that came with it. When she, like the OP, goes off to her big research institution for her PhD she will have had lots of the benefits of an LAC experience and be ready for something different. Her cousin (ok, sample size =2) went to JHU knowing she wanted to do science research, and loved it for all the opportunities, the competitive peer group and research opportunities. Horses for courses.