Swarmkeeper
Hero
I think part of my dissonance with the whole theory of having a low level character joining a high level group and leveling incredibly quickly is that levels are supposed to reflect roughly learning, training and getting better at your job through experience.
I see where you are coming from, but what better experience for a new PC than to adventure alongside some mentors who are further along in their craft? In a fantasy world, I could see the low-level adventurer learning REAL quick when thrown into the fray of some higher level encounters.
To me there would be no sense of earning those levels, especially if I'm a front-line fighter who does nothing but hide in the back. Since I don't really "earn" those levels, why bother? It feels like filling out unnecessary paperwork with no value in order to be official.
A very combat-centric view, to be sure. In our game, we award XP for exploration and social interaction too. If the concern is combat squishiness, the character certainly can "earn" their XP contributing in the different pillars of play, while doing their best to not die against deadlier foes in the combat pillar.
Another point to consider: Are the monsters that the party faces in combat all the same level? That would be a dull way for the DM to operate, IMHO. Now, I'm not saying that a party of same-level PCs is dull, but it certainly isn't the only way to conduct a "realistic" campaign.