True. It only managed to send the fabulously futuristic Spira back into the Middle Ages,
That has to do with the Yevon religion ostracizing the Al Bhed and banning all technology.

keep it there for 1000 years,
Thanks to the lies of the Temple.

destroy any settlements that grew too large for its liking
Bevelle seems fine.

and accumulated a kill count that almost nothing in the whole history of fictional stories can match.
With a longer existence than nearly any other FF villain, Sin accumulated probably the lowest kill count of any of them.

Oh didn't he?
Yep. Flatly said Bevelle did it and possibly Unalesca.

Or do you think Jesus created Christianity because people spread his teachings and formulated religions around him?

n any case, Sin wouldn't have destroyed Luca, as that would've eliminated blitzball and with no entertainment, the people would lose hope. And not only does Bevelle have the protection of the warrior monks, but has a temple that summoners need to pray at to get the Final Summoning. No Final Summoning, no hope.
The Final Summoning kept the people slaves to ignorance and fear for a millennium.