As someone who works in academia this is something I'm always amazed by, this notion that your intellect is set at 18 and consistent thereafter. I have encountered students who had poor A levels but a different environment at 3rd level led to them excelling in university, PG study and in the professional field. Others who crushed their degrees but couldn't function in a professional environment due to a lack of leadership or chronic indecision.
Intellect like fitness, is something you lose if you don't use. Intelligence is great, but intellectual curiosity, mental toughness, humility, persistence and dedication will take your further.