Part of it is due to the game design though. There's a non-zero amount of games that are lost not because of skill, but because of another player trying out a n

Part of it is due to the game design though. There's a non-zero amount of games that are lost not because of skill, but because of another player trying out a new hero or build, someone having an off day, poor team composition, or the team chat degenerating into an all out flame war. If that happens, there isn't much that can really be done, and one is stuck in that doomed game for another half hour of unpleasantness. That's definitely a frustrating experience. As you point out, the ELO system is fair, but it's also a rather noisy metric. Players can be skilled with some heroes while being completely awful at others, and the ELO rating is only a singular value that has to capture this range of player ability.