Having a guitar to practice on is a mandatory part of learning to play guitar. Having adventures to run, be they modules or home made is a mandatory part of learning to DM.
I didn't have any modules when I learned to DM. Somehow I managed.
But guitars don't teach you how to play and modules don't teach you how to DM. <snip>
The DMG is like a book on "how to play guitar." (at least key parts of it)
What they could probably usefully do is a series of modules for beginners that incorporate those bits of the DMG that are relevant to the module
right into the module. So, instead of putting "how to run encounters" in the DMG and then having the module present just the encounter (and assuming the DM is familiar with the appropriate bit of the DMG), have the module present the encounter
together with the relevant bits of "how to run encounters".
Of course, such a module would necessarily have a much larger page-count than the equivalent adventure that doesn't do as much "hand holding", so it's best suited for those beginner adventures. So that does suggest that these should be a separate, dedicated product. (Or perhaps a "DM's kit" including several such modules, where the DM who has run them then 'graduates' to the full DMG (also included).)
It's also perhaps worth noting that WotC did
try to do a module for beginner DMs, "Scourge of the Howling Horde". It failed, but it's not clear whether it failed because of the
type of the product, or if it was because they screwed it up in just about every single possible way they could (
and chose that product to debut their new, higher, prices).
But, back to the start of this: Do you think that Tomb of Horrors is an example of a problem that contributes to the creation of bad DMs and for which the publishers should be blamed?
Yes and no. "Tomb of Horrors" is module S1, that is "Special 1". It was a particular type of module intended for a particular type of use, and in that use, it's fine.
Where it goes wrong is if it is mistaken as just any old module, to be used by any DM for any purpose. It's
not a module for newbie DMs, and it's probably
not a module to try to insert into general campaign play. It's also
not a module that should be used as the general pattern for module construction going forward (unless you're explicitly trying to emulate that death-trap mentality, for the same
special use). Used improperly, it can indeed contribute to the creation of bad DMs.
And I think that's where we've gone wrong, that the uniqueness of ToH wasn't highlighted as clearly as it should have been, and that it's been taken as a general guide. And that's something the publisher(s) can be blamed for.