Hime obsolation and power creep is an interesting point that i didnt consider. Will that happen? I didnt check jp wiki and its hard to imagine himes like Amaterasu/Sol/Ryu-oh/Cthulhu get replaced. They got their place in core hime by bringing immense amount of utility. To replace them they would need to release even stronger himes like with 30-40% debuff or himes with 4 active ability, otherwise new himes doesnt replace them, just give an equal substitute.
As for dmg himes, yeah its pretty realistic new dmg himes get even more dmg and new interesting mechanic, but i'm more interested in utility when i build teams (in all games). I will opt for versatile or endurance build if its possible, and only go burst only if the game force me to do.

In conclusion: 100% eido isn't a free pass to endgame content, but gonna make your progress smoother. Multiple core himes of the same element is also a nice way to start out, but that's probably even less likely than a 100% eido of RNG's choice & yet, those himes will fall out of meta sooner or later.
You missed my main concern. Im not concerned about easy or hard way to endgame. Im concerned about bottleneck when i reach endgame in the far future. I understand 100% eidolon is indispensable, coz its the only reliable way to boost elemental dmg. Does SSR hime also indispensable for a good team, which force me to reroll for both, coz otherwise there's no garantee i get SSR hime for endgame?
As for the definition of "good team", im obviously not planning to compete for top ranks as free player. I would consider a team good at endgame if it can comfortably earn all individual (not competitive) rewards for event, and clear all solo content.
I guess either way i'll reroll 200 more just to reach nice number (currently at 800). Thanks for reply.