I've disliked "holy" and "unholy" as it draws us into the rather muddy waters of a dualistic monotheistic religious system, ie: real life and Christianity.
Dualistic? Not by any definition of the term I'm aware of. The only major dualistic real-world religious system I'm aware of is Zoroastrianism.
Those things don't really exist in D&D, so I feel that "radiant" and "necrotic" fill the voids better because they don't harken to a particular religious ideology.
I don't think monotheistic systems have a monopoly on the idea of holiness or the sacred.
And for that matter I have created monotheistic D&D campaigns in the past; I know I'm not the only one. It's not hard at all - you just have clerics of different Domains being devoted to different aspects of God. They certainly don't have to all get along like one big happy family, either.
Now what *is* rather strange about many D&D settings is that they often feature polytheistic pantheons in cultures that just don't seem to reflect a polytheistic aesthetic or outlook in any other way.
"Holy" presupposes "good" and "unholy" presupposes evil, but traditionally in many societies, sun gods were considered highly wrathful and destructive, giving rise to hot summers, wilting crops, and drying up sources of water. Likewise, gods of death were not always framed as murderous monsters and the powers of death were as natural as the powers of life.
Okay. So, if that's what you're going for, why not make your sun god of evil alignment and use unholy damage? Or just avoid those spells entirely and use radiant? But when it comes to a guy like, say, Lathander, holy seems to fill the bill.
I'm right with you on the ridiculous way gods of death are treated by pulp fantasy generally, and D&D in particular. But it seems easy enough to fix the problem by not making them evil-aligned. I don't think it's particularly helpful to give them necrotic damage, though, since that has knock-on effects with undead that a god of death probably disapproves of.
(Of course, that just gets me started ranting on the illogical ways undead have been treated in D&D for a long time... To name just one thing, why can't the Inflict Wounds spells just be expressions of divine wrath, and not heal undead at all?)
"holy" and "unholy" is more like alignment. It's all about perception. Radiant and necrotic are actual power sources. Evil beings can perceive themselves to be holy, holy beings can wield necrotic energy.
We needn't go all relativist. Though if you want to, even that can be fitted into a holy/unholy dichotomy. Like I said, just view holy and unholy as different flavors of 'divine' - it's the power of The Gods. Devils, at least, have fluff of formerly being divine servitors; and some gods side with them now. They're 'unholy' - the side they rebelled against (some of which might even be evil-aligned, if you want that) is 'holy'. On this view, 'unholy' is synonymous with 'rebel', for better and for worse.
Undead could fit into 'unholy' because they are products of disrupting the ordinary process of death that the 'holy' death gods represent and uphold.