I'd like to turn this argument around a bit, because the philosophical "what is a 1st level PC" distracts from what is, IMO, the main issue - "what is the
purpose of a 1st level PC?" That is, what's a 1st-level PC
for?
My answer is, "It's an introductory character which will be fun and easy for a new player." Ideally, I think this should be a character that will grab a brand new player's attention, help them have an entertaining session of D&D, and that they'll care about enough to advance. Having a character who dies in their first skirmish with rats is a big, giant wall thrown into a newbie's face.
This fits into the "swinginess" argument above. Fewer random, brutal deaths at 1st level is, IMO, a feature and a draw
for new players. I've been playing D&D and other systems for close to 30 years. I get the appeal of ordinary, fragile, and weak characters, and I think there should absolutely be a module for this*. But when a new player - a new entire
group, learning D&D together - is just starting out at 1st level, I think the game system should be encouraging rather than discouraging. And today that means a character capable of doing neat, interesting stuff influenced by fiction, movies, and better television. And which won't die immediately for one mistake or bad die roll.
Saying, "Just start at higher level!" misses the point that when players new to the game dig in for their first adventure, it's going to be Level 1 PCs they're making.
-O
* my own "module" is called WFRP2e