Hendell:
There is no reason the artificer can't get into the fray. And indeed spellcasters of all kinds can easily take the primary role in almost any situation if they are willing to expend a few limited resources to do so. The question is do they need to be in everyone's way doing so?
In this case, the spellcasters don't seem to be "in anyone's way." The artificer seems to be acting appropriately for a game about investigation and intrigue. Perhaps they'd be acting differerntly if the rogue had been present from the beginning, and perhaps some artificer players would be relieved at the addition of a mundane detective, which would free them up to prepare other spells. If the artificer is enjoying being the investigator, though, and is good at it, they're helping, not hindering. They might not be helping optimally, but they're helping in a way they (presumably) enjoy.
Hendell:
If your whole party can manage stealth then you can do that, but sometimes a quick glance at the setup of the room is worth a few minutes wait. You don't want the Rogue to go full solo stealth mission and sneak through to the quest objective, loot the place, and come back out. But pop around the corner and get the room description, hand that off to the other players so you can make a plan without being jumped by the angry things in that same room.
The whole party can manage "stealth," depending on how one plays things. A skill challenge that's about infiltration can probably involve every character in some way. Rogue types would make the Stealth and Thievery checks, strong types would handle climbing and abseiling, the knowledgable ones can guide everyone, etc. Ideally, of course, there's more going on at the same time than just "infiltration," maybe "avoiding guards" is split out as its own thing, or there's some secondary goal like finding and grabbing a particular item.
Hendell:
It is part of what makes Rogues different from other characters and eliminating it would be very similar to eliminating the combat in a game and being surprised that the fighter isn't particularly useful.
Except that pretty much everyone can participate in combat, and probably expect to be able to do so to about the same degree and with as much enjoyment as the fighter.
Hendell:
As for making magic difficult that isn't an important step, just insist that the spellcasters use and track time, 5e spells have a very short duration and most classes don't get a lot of them, so if you keep track they won't be active nearly as much as they were in 3.5 games.
I'm glad you said "time" rather than "components" which is the other thing people seem to point to when trying to keep casters balanced with non-casters. Yes, tracking time can help with keeping options balanced, but the fact that you have to bring it up, rather than it being an inherent thing to track like HP or spell slots points to why it's not a reliable way to balance things. Besides,
enhance ability apparently works for an hour. The example you give of the rogue popping around the corner implies something quick and easy, well within the duration of that spell. Now, maybe someone would prefer not to use a spell with an hour's duration on a quick recon, anymore than they'd want to waste a fireball on a single weak target, but with an hour to use, they could scout, come back to plan, and probably still have some of the spell left for use during the plan, as long as the plan didn't require more concentration spells.
I'm not clear whether casters can make scrolls or wands in 5th Edition. If anyone could, I'd expect it to be the artificer, but maybe there are adequate downsides to prevent those from obviating spell-slot limits.
Frankly, enhancement spells are are primary issue here. The game is better off without things that allow someone who is a good back-up in an activity to move directly into the primary for that activity, for any significant length of time.