You've admittedly had a rather small sample size (2 occurrences) to extrapolate a drop rate.
The frequency of DTDs is closer to 1/month. They happened to fall adjacent on this occasion.
I can understand the perceived value of a deathtouched dart since it has the potential to be a big drop for minimal effort. But it's effectively a lottery ticket with extra steps.
Compared to, say, direct coin creation like a cash bag (which, BTW, we're dropping a size tier and substituting for ironmen with oddments in response to feedback), a DTD's one of the more economically stable rewards we can give - at least so I've been assured.
Boss value is measured in kills/hour and bosses are balanced around being repeatedly killed, so one free kill through a throttled non-tradeable supply that requires daily engagement to acquire contributes minimally to inflation.
It's an active reward - you have to gear up and fight the boss (albeit briefly) to get the benefit, so they're actually closer to an onboarding tool. They don't work on several bosses, have a lot of restrictions (I'd give Mod Raven's left testicle for them to count towards Reaper Crew - frickin' group bosses) and some of the most valuable bosses only skip a phase.
We even checked in with the ironman council yesterday, and after verifying the reward frequency to be sure, got confirmation they're happy for DTDs to be given to ironmen at the current rate.
Please do keep giving us your feedback about daily challenge rewards as they present, though, and we'll keep an eye on DTDs. We want to keep tuning the daily challenge rewards so they feel good to you.
Thanks for coming to my STED Talk.