@SKyOdin
If you choose to limit the number of adventurers to your PCs plus some foes, that's perfectly reasonable but that's hardly how most people are likely to see it. This is what I mean about setting a common baseline
So, your view is consistent, reasonable and useful but it does seem a bit of a stretch to tell me I'm wrong because of this previously (to my knowledge) unstated way you handle it in your settings.
Regarding using earth magic/alchemy as an analogue for a magic world, I think the problem there is that earth magic/alchemy is either entirely powerless (my personal view) or of very limited, measurable power (allowing for other view points on this). D&D class magic is so much more patently powerful I don't see how you can use it as an example. I'd pick instead access to medieval universities or ancient world engineers; rare but not unheard of it.
I'm up for another thread but, if I may suggest, please take a stab at some ground rules to the discussion. Someone can always start another thread if they don't care for the ones you pick
I'll take a stab at my own thread regarding a premise similars to yours which I described as a "distortion bubble". I think it does have it's place.
I was talking about access to medieval universities. I could cite comparisons to ancient/medieval Chinese education too, since it followed the same patterns: only the wealthy elite had access to any kind of advanced education, and only a fraction of them succeeded at it.
Anyways, once again I reiterate that my argument that has never been that it is wrong for people to create a setting with an advanced magic-industrial revolution or a setting full of high-level characters. I have no problem if you want to create a setting where every street-corner has a ten-thousand year old everburning lantern on it, or a setting where war is fought between small, elite armies of 15th level magic wielding ninjas (I am a Naruto fan, after all). I am just arguing against the idea that it is impossible for a DM to adjust things to make these outcomes impossible. I am also arguing against the idea that these outcomes are an inevitability of the D&D rules system or that such settings are a natural progression of fantasy itself. My main point is that if you want to avoid it, you can do so quite easily.