No, I don't think it would have been.So if the design aspect being a DM was new to you, you'd not have found discussion about design choices to be useful?
If I'm explaining how to (picking random subject) write an SQL query I probably don't discuss how the data is stored on physical pages, why they do it the way they do it or the history of databases. It's not particularly relevant.
D&D, especially current edition, is not just 1 or 2 people sitting down and deciding what's best. It's a committee that proposes ideas, sends those ideas out to a relatively large playtest group and then does a broad based survey. It's a different process than most TTRPGs.
Sure, you try things out. I'm not talking about just the book in a vacuum. Yes, play things and try them out... and I'm all for people diving in and trying to GM games as soon as they can, and to not be afraid to do so.
The book doesn't exist in a vacuum though so it's not a relevant assumption. It's not like ye olden days when a significant percentage of new players just found the game while browsing in B Daltons.
While I think the books are there for them. Starter sets if you don't understand the basics, there's a fair amount in the core books for brand new DMs.But I think that the books should be there to assist them with his. Not to be saved for some future time when they're experienced enough to be a "real DM" or whatever.
I mentioned DMs of all experience levels. Sure, the full context of a specific bit of design advice may not be apparent to a new DM, while being insightful to an intermediate DM, and perhaps a good reminder for an experienced one. But there's no reason that such advice can't say this along with the advice.
As with any instruction, you start with the basics. I think basic design advice would absolutely suit the starter set or essentials kit. That product is specifically designed for less experienced folks, so of course it makes sense to cater to intended audience.
Then, build on those things in the DMG.
And when one is done with the starter set, one is then a fully competent DM?
I've been DMing for decades now, I'm still not a fully competent DM.
Like, at what point do you think the DMG should be useful to someone DMing?
I think the DMG is already useful to anyone who has a decent grasp of the rules.