True, but the Fallen seem to tailor their temptations to the individual person that they're tempting. Lasciel's shadow, for example, offered Harry knowledge, magical power, and help in protecting innocents. She did not offer him a way to become as beautiful/handsome as Thomas and unlimited wealth, because that wouldn't have been particularly tempting for Harry.
The Denarians have an agenda in the world, and it's NOT collecting the souls of their bearers. I believe they preferentially target those who already have an urge to power, toward being effectual actors in the world, because they can leverage that to their own ends.
That said, they have no difficulty in playing a "long game" for years or decades (or centuries!), or going with shallow vessels who have trivial wants and needs, and no "leverage" in the world: the demonic equivalent of a short "hookup" with no relationship!
But 1) there might be other ways that Harry doesn't know about--it's not like he's done much research on the subject
True enough. Canon never says you CAN get hellfire another way; but equally, canon never says you CANNOT. (also, even if Harry had an opinion in the books, we KNOW Jim Butcher uses Harry to "lie" to the audience -- Harry is perfectly capable of being wrong (in fact, he figures out at least one "wrong answer" in several novels, before he finds the right one...) I would undoubtedly consider Hellfire-without-Denarians a per-table / per-campaign consideration, leaning toward "what do the players think would be most fun?" (noting that as
players-plural, because others at the table might have strong opinions about hellfire; no one player's concept should ruin another player's fun).
and 2) I'm talking about a character being tempted by a blackened denarius, so I'm not sure how this is relevant.
Maybe I don't understood. I thought your suggestion was that they somehow had the goodies a Denarian could offer, so why pick up a coin?
AFAIK: if they never took up the coin, the Denarian within it cannot tempt them (nor empower them).
I think those powers are a big part of the temptation to take up the coins (which make it seem like other channels to gain hellfire are likely to be harder to find, harder to learn, etc).
You want insta-power?
Here.
Or you could go study for a decade or more under a master you can be CERTAIN is corrupted by Hell itself.