two words: Cover up.
The gov't/blamer might have gotten away with saying Archer did it, and pulling strings to hide the truth. After wards, he just encouraged some rumors. A century or two later when the next grail war rolls around, he's summoned because he's the insane and mysterious villain/hero with twisted ideals that launched a war and slew ten thousand people in order to save a handful. mix in a slight apocalypse setting, or an excuse for humanity to lose a lot of knowledge and information, and bam! you've got your fame.
.........
Disregarding how utterly cliched and just... ugh, sounding that is...
You're not getting it. It's not a matter of time difference, it's a matter of the way information is distributed now versus back in the past. It's not possible for Eirei to be created in the 'regular' way of gaining fame since information about them is too... accurate, I guess? And no, misdirection, rumors or what have you don't mean anything.
I
suppose if the apocalypse set back the human race something like a millennium in terms of technology and knowledge, it
might work, but honestly, that sounds just silly. Not to mention that if something like that happened, all the famous heroes, let alone a nobody like Emiya might not even be remembered. It's a scenario that I don't think is really logical, since it's more likely that any sort of disaster like that would wipe out mankind period, not the technology and knowledge.
Also, Angel Notes.
What happened to the Grail system after the war, anyway? I know the grail was the 'main' piece, but still...
Also, couldn't they, theoretically at least, create another grail war? You know, start from scratch?
It's dismantled or destroyed, depending on what route you're going from. In HF, the cavern collapses the Greater Grail, (and if you go by the True End, Ilya closes the 'gate', which means it wouldn't even be possible to try and start another system at Fuyuki). And in UBW+Fate, El-Melloi II returns and takes it apart.
In other words, after the fifth war, there AREN'T anymore Grail Wars. I somewhat doubt the Einzbern's would be willing to lend their hand in the creation of another system, either.