I don't disagree with anything else you said but I don't assume that magic is just another type of technology. Another view is that magic suppresses technological advancement. If magic can, at a cost, transform lead into gold it explains why the currency is gold instead of silver. But it also means that there's less need for experimentation and attempts to transfer lead into gold using chemistry. Because those experiments didn't happen, knowledge of chemistry doesn't advance. We don't get advances in firearms because the early firearms would be inferior to magic in just about every way. Diseases? Well, some diseases are supernatural in nature and there are magical elixirs that work so why fall back on inferior medical practices? The best and the brightest are going to focus on magic as a solution because it works.
I don't really follow this. I said "magical innovations" not "technological innovations". I know perfectly well that if given one solution that's known to work (magic) it's not very likely that other solutions (chemistry/medicine/etc.) are going to be looked at quite as seriously. However there
is a Medicine skill, which implies people at least know enough non-magical healing for it to be a viable option. There are also regular tool proficiencies, which implies people don't see magic as a solution for
everything.
Plus there are people who study magic: Wizards (especially Order of the Scribes) and Artificers. While 5e seems to be very unfriendly towards the creation of new magic items, from an in-world perspective there ought to be quite a lot of study on the matter. Armors were developed to counter various types of weapons: weapons various types of armors. People are constantly looking for ways to get the upper hand in battle scenarios.
Since at least the Industrial Revolution, there has been a tie between military advancement in tech and the eventual use of that tech in civilian life. Now, a
magic Industrial Revolution (similar to Eberron) might not happen, but people would indeed be using magic as a solution and probably try to be innovative with it.
Then there's the flip side. Perhaps magic just has hard limits that technology eventually would have surpassed if advancements had not been cut off at the early stages. It took centuries of development to get to the level of technology of flintlock firearms that most people associate with black powder weapons. Who is going to invest in a steam engine when there are semi-autonomous golems running around? But there's no reason to believe that magic could have gotten past the level of effectiveness that we see in the PHB. There may be no way to manufacture large quantities of wands or harness elementals to run your train like Eberron.
That's also true, and this can be very campaign-dependent. However, there's also not really any good reason it
couldn't either. The trick, mostly, is figuring out what exists and how it works. The mass manufacture of items is certainly where the limitation comes in. If it becomes quick and easy to create magic items then it also means the costs of those items go down. The balance issue becomes one of dealing with folks who have what they need for most scenarios rather than the starvation rations that is more typical of D&D.
When magic can literally do miraculous things, why would people work on creations that will never rival the proven abilities of magic in their lifetimes?
So, I gave examples of spells that would likely give the same effects as the weapons in question both to show that (from a game perspective) they don't
have to be over-powered but also that there are already spells that could give the same effect. I don't think firearms (as we recognize them) would be developed because there are already spells that do the same thing. However, that doesn't mean they
couldn't be developed. The Artificer (Artillerist) comes immediately to mind for this.