One Subclass is the whole Class. More are not required. Mearls also said that literally all of the crunch was likely to be repeated I any future hardcover, which came to pass. We know that not all of the WGtE is being reprinted, per D&D Beyond, so I'd say keep an open mind.
The artificer will be included in Wayfinder’s once it’s complete. Just because more Artificer subclasses will exist in another book doesn’t make it “part of the Artificer” any more than the existence of the Arcane Archer in Xanathar’s makes the PHB Fighter “part of the Fighter.” Or for that matter, the existence of the battlemaster in the PHB makes the Fighter in the basic rules “part of the Fighter.”
In general I agree with what you all are saying about WGtE, but I think I don't think this is right. It doesn't feel like good faith, it feels like "WELL TECHNICALLY!!!!" rather than honest intention. Every other class in the game has launched with at least two subclasses. You can't "choose your subclass" if there's only one subclass. On top of that, it feels like a double-technicality because they made the Artificer UA rather than putting it technically in WGtE, presumably partly in order to get away with this.
What I'm saying is, WotC have earned a huge amount of respect from me with most of their 5E decisions. I've never felt like they were trying to rip me off, or doing something cheap or "technical". Indeed, that was rarely the case in 3.XE or 4E.
But this feels like total money-grubbing cheapness of a disreputable, video-game-publisher kind. It genuinely lowers my respect for WotC and their business practices, and makes me not want to buy the new Eberron book (fortunately I probably don't have to as I think I'll get access via content-sharing on D&D Beyond with my DM in the Eberron game).
Comparisons with the free basic set are obviously not right. This isn't a free product, and they specifically promised the artificer. I could see including only 2 of the 3 subclasses, just so the new Eberron book had a bit of additional content, but only one? Rude and feels sleazy. Is it worth making my opinion of you go down WotC? Really?
I feel like an NPC in a computer game when the player selects some really ill-advised dialogue option and my opinion of them drops 20% on the reputation bar or something!

Like Tyranny maybe "You have gained Wrath with Ruin Explorer" and WotC flips to the reputations page and sees they just went from like below 1 Wrath to over 3.
TLDR: Morrigan disapproves