I believe you that you've mentioned previously you were speaking sticking to what we know from the playtest document. In the post I was replying to however, there was no such explanation.
Yes, I neglected to put that disclaimer in every single post in the whole conversation. I tell you what. Pretend I changed my sig to say, "All references to playtest material are for lack of any better data, and with the understanding that things may change in the final rules." That's in every post I ever make from now until all three core books have been released. Okay?
I agree, if there is a compelling in-fiction reason for fire immunity in the first place.I do think that penetrating immunity is a bad idea - because it just makes no sense you can do something to make something immune to fire, suddenly harmed by fire. If they were to change anything, it should not be to make things less immune to things they are immune to. That's just purely gamist and doesn't make sense to me.
The problem with 5E's approach to immunity and resistance (and the reason I suspect we won't see immunities scaled back much in the final rules) is that there is no longer a simple way to say, "This monster is immune to ordinary fire, but can still be hurt by intense or magical fire." In previous editions, you could do that with something like fire resist 10. With 5E resistance, though, even normal 1d6-per-round fire will hurt and eventually kill a fire-resistant creature. So when you have creatures like devils, fire giants, and red dragons--all of which tend to inhabit fiery environments, but could plausibly be hurt by super-intense fire--you have to make them immune to explain why they aren't dead.
IMO, there needs to be a way to distinguish between "basically immune" and "really truly totally immune." If we see immunity in the final rules to the same extent as in the playtest, I will probably implement this distinction by house rule in my game: Anything that lets you bypass resistance, also lets you bypass "basic immunity." So EA will let you deal fire damage to a devil, but not to a fire elemental. If there's a poison equivalent, it will work on green dragons but not undead. Et cetera.