Kirby certainly claimed that the FF were basically his idea and Lee just touched them up, but Kirby has been known to exaggerate in anecdotes (look up one where he found Lee crying in the troubled Marvel offices in the 1950s as furniture was being removed, and heroically reassured him that he would save the company - an exchange Lee denied ever happening). Also, they supposedly used the "Marvel Method", where Lee created a synopsis, Kirby illustrated the story, and then Lee came up with the dialogue. So a version of Kirby's pencils with suggested dialogue doesn't contradict Lee coming up with the story. (There's more on this in Wikipedia - https://en.wikipedia.org/wiki/Fantastic_Four#Origins
Not to mention that Kirby's subsequent solo works (such as the New Gods), while well-regarded among us comic fans, didn't exactly reach the same popularity among casual readers as his collaborations with Lee. So I'm pretty sure Lee added just enough of a special something to make it work.
Anyway, fair point that this is what the entertainment landscape would look like in the MCU itself!