I do see your larger point regarding the fact that the usefulness of different elements for end-game content is not at all balanced (though I wouldn't say it's just elements; claymore users [and Klee] are also simply easier to play as carries in my experience since they inherently break shields).
However, there are people who have cleared the entire Spiral Abyss with only the completely free characters, so not being able to try is not completely true.
Claymore users do have an advantage, but a bit less so now with cryo hilichurls (as their shields regen too fast... except against pyro, yadda yadda)... though like elements, it does pose a question: one weapon has an inherent strength against geo constructs (and wood, but you know, not relevant), all the others have only a weakness when fighting it and no advantages whatsoever. Again.
May as well just say that the entire game was made for Diluc.
But it goes even beyond just the power of the element and how useful pyro can be, but also that is has the best resonance by far. Then cryo gets second place.
Hydro gets you increased healing, almost useless. Electro gets you 1 more energy per 5s, factually useless as it won't even be used since you have two chars generating it to begin with.
The two best elements get the two best buffs when you double up on them, and get even better if you combine them.
The two worst elements get two useless buffs, and they don't even work well in tandem. Or with anything else for that matter.
As far as the Spiral goes, sure, some people finished it with only the free chars. Barely, with almost no stars, because the old model required two pyros, two cryos.
Now it's probably more in line with heavy, heavy control... and if you can't even have Sucrose, I wonder how it goes with only the MC. The free blokes are not good for what's required. And the solution being "exploit the AI" certainly shows even more how broken the systems are.
But that's beyond my point, as any game can be done without getting hit, naked, no continues, etc, etc. But that requires an absurd amount of time invested in a game, even more so in a grinder like Genshin where grinding for very poor chars will literally cost you.
"But speed runners and youtubers can do it when it's their hobby/livelihood" is not defense of horrendous balance.