How long would you say it takes to gauge a parties capabilities? If you say the first pull then we're right back to the argument of we shouldn't judge people by a single solitary example (a la logs). If you go by the notion of up to the first boss, then over a third of the dungeon is over and who honestly cares at that point.
I'm an optimist. I very firmly believe that if you challenge people they will rise to the occasion. Worst case is that we fail. Failing is not a bad thing. It teaches us things and prompts reflection, which is integral to growth.
Me personally? No I'd brush it off with something like: "I apologize for wiping us, I thought we could have handled that."
As I said before, I'd rather a new tank do this and fail, then sit in tank stance, tanking one mob at a time only using 1 combo because they're so scared of losing aggro. Sure he'll succeed (because participation trophy content), but I'LL be sitting there ROLLING my eyes and be annoyed at the 32 minute long dungeon run that should have been half that.
It's possible for sure and I won't pretend to speculate either way. I merely gave an anecdotal experience to paint a picture.
I'll give another IRL example, that is a little outside the relevance, but the lesson fits. I was roughly 16 years old at a 3 week tennis camp. I was given a choice based on my skill level. I could be placed in the intermediate group and likely be untouchable by 90% of the teens there, or I could be put in the advanced group with the top players in the state. I was told that it won't be fun, and I'll likely be dead last and lose every match/game (hint: I did).
In this situation (and I didn't really understand it that well then) I had to choose between my participation trophy or growth. Just like you can choose to get your trophy by completing the DF run at your slow careful pace, or you can grow as a person as push yourself and see just how far you truly can go.
I ended up choosing growth because as a competitive person winning something that is free, isn't winning. It needs to be earned. You'd be surprised just how many of these pugs surprise themselves.
You are welcome!