Dear Dean,
Thanks! Parenthood has been amazing so far. It's not lost on me that as a new father I am making a lot of claims about heavenly fatherhood.
Regarding the claim that "the importance of a dyaus pitar can't even really be reconstructed to the Indo-Iranian level," what I mean is that Iranian doesn't provide any evidence for us, so if Dyaus Pitar was an important figure, his importance likely declined already in a shared Indo-Iranian period prior to the speech community's division.
I am not certain there is particular importance, personally, to assign to the figure of Tiu. Consider Old English Tīwesdæg "Tuesday" is etymologically cognate with a hypothetical Sanskrit devadaha*, which I think is the name of the great-grandfather of the Buddha (via Devadaha > Anjana > Maya > Siddhartha), if I am not mistaken. And in Irish, the Dagda would go by cognate, hypothetically, with a Sanskrit dahadeva.* My point is first, Tiu goes back to deva not dyaus, and that's a real difference. Second, it's possible to create what looks like a PIE figure from Tiu, Dagda, and Devadaha but I think it would be ill-advised to do so.
BUT In an email to me, John Lowe pointed out, and I think quite convincingly, that we have to reconstruct a vocative Dyews Phter to the proto-level---which is suggestive of a certain use, isn't it? An atheist today might well say "o god!" without believing in god, simply because the vocative is an inherited part of the lexicon. So, this pragmatic dimension does make me reconsider what I said earlier, and I thank him for pointing this out. Still, this figure would likely not resemble, in my mind, the Greek Zeus and the figure of Ouranos looks to NW semitic to me for a reliable reconstruction (cf. Ugaritic El, sky-father of the gods, who are conceived of as the stars in the sky), when this astrological element is absent from Sanskrit Dyaus Pitar (although Varuna does have night-sky affinities...). I'll have to think very carefully about this.
As for Indra's replacing Dyaus Pitar, I am not convinced. I do think a sukta-era Indra-ism did result in the appropriation of a broad spectrum of heroic deeds to Indra, but in this case it's more likely that Indra is taking over mythical deed once attributed to Trita and Mitra, this there is some comparative data from Avestan that let's us think through the problem. But what is the evidence of IIr Dyaus Pitar actually doing anything?
Conversely, let's consider for a second what we know about the sukta-era political system and the function of the Soma pressing ritual. It seems like it was used to consecrate a temporary leader, we might think of this leader as suzerain but I think kingship goes a bit too far in terms of sovereignty. This kind of power was likely based on a patron's personal charisma legitimized through priestly/poetic performance and ritual distribution of wealth to shore up support. In other words, part of a process of coalition building. Any coalition would have old leaders and new leaders, and not necessarily ones from the same clan let alone household. It stands to reason that a mythological structure that provided a conceptual map for the peaceful transition of power would be completely appropriate in such a system. This is something Kuiper, Jamison, and Brereton observe in X.124, the peaceful switch from asura power to Indra's devic supremacy. Put differently, we need not imagine that a deus otiosus ever needed to be, once upon a time, the primary object of worship. His role could always be that of deus otiosus going back as far as you need. It's unmotivated to assume the necessity of mythological replacement, when there is real social value in the figure of a "sky god emeritus." In fact, in Ugarit this is exactly how the god El functioned, Baal was the primary object of worship and El was a creator/fertility entity whose primary job was to resist and then accept Baal's supremacy.
Not that we should reconstruct PIE to be like Ugaritic! My point is that a myth like this could serve a social function, while different systems of political organization, for example polis kingship in Greece, could reinvent the figure by fusing elements of a deus otiosus with a heroic actor (thus Zeus, this parallels the figure of Yahweh who appears to be a fusion of Baal and El).
So, at the risk of seeming strident and insistence (and I don't actually have a real theory of Dyews Phtr just qualms), I think what the figure meant and indicated in PIE is still very much an open question, but if I were a betting man (and thank the gods I am not), I would ask myself what the political organization of steppe peoples is like and if it is more like how Indo-Iranians were organized or more like how Greeks and Romans were organized, and what kind of myths would make sense to the worldview of that audience and would be useful for their own political operations. Just thinking out loud!
Best,
Caley