This seems to be veering into tiresome nitpicking and willful misunderstanding, but I'll give it one more go.
I suspect you already understand that novelty is not the only goal of creative work, so I'm just going to go on past your first bit. Let me know if you truly don't get it.
He didn't say that debuts are usually the best art pieces. One, you have the flow wrong. He's more saying that the best art pieces were debuts. But two, he's not saying that either, because "things" is broader than art, and "best" is different than "really amazing".
As an example, look at Hamilton. I'd call it both amazing and great. But it wasn't done by somebody with decades of training and experience. Miranda started work on it when he was 28. It was not his 10th or 20th rap-influenced historical musical. It was his first. And only his second produced musical period.
The good thing about experience is that it gives us habits that work quite well. But that's also the bad thing, because it's very hard to approach the top fresh, to do enough things that don't work on the way to finding something novel that does work. So a lot of great creative work is done by the relatively young and inexperienced. And as Kelly says, we should give them the room to try.
The flip side to your example is, of course, artists like Rodgers & Hammerstein, Sondheim or Bernstein, all of whom produced good works in their youth but some of their best works as they gained more experience and developed more style.
This is how one goes about understanding (or extracting a meaning from) a point. By poking at it with scepticism and watching how it holds up to scrutiny.
That is one way to do it. But if you expect somebody else to participate in your particular chosen process of understanding, you'd really better demonstrate some actual understanding, and perhaps some gratitude for their gift of labor. Because from here it looks like garden-variety tedious argumentation based in willful ignorance, which is everywhere on the internet.
Of course there are Rimbauds, and there are Monets. That's beyond dispute. The implication I'm not sure about is whether the former outnumber the latter.
It's psychologically understandable for someone who's accumulated considerable life experience to feel more nostalgic towards the freshness of being unexperienced.
Isn't it a tautology then? If the goal is to make something you haven't before, then obviously it's accomplished the first time round, by definition.
And in a broader sense, is it really true? Are debuts usually the best art pieces?