Just give it a TRY damn it!

[Hambone]

I'll never understand where people get their ideas on Yu Yevon from.

For one thing, he clearly wasn't a tyrant as every single one of the remaining people in Zanarkand willingly became Fayth for the summoning of Dream Zanarkand.

Sin only exists because from within Sin, he could perpetually keep Zanaraknd alive "if only in a memory" in the form of Dream Zanarkand. it had nothing to do with ruling forever or creating a Spiral of Death.

Finally, it was Yunalesca, not Yu Yevon or the leaders of Bevelle, who did what you said.

"On the eve of Zanarkand's destruction, Lady Yunalesca... had fled to safety with her husband, Zaon. Later, the two used the Final Summoning to defeat Sin. Yet the people of Bevelle still feared Yu Yevon.
It was to quell his wrath that they revered him, and first spread his teachings.
And so were born the temples of Yevon.
I suppose it's possible Yunalesca had planned it that way from the start!
A fair trade, she defeats Sin in exchange for her lord father's honor."

Even all of that given, my point remains the same: Yevon was happy keeping Sin around because it meant that Yevon would remain prominent and unchallenged throughout Spira.

Where does it state that the people of Zanarkand willingly became Fayth? According to the Final Fantasy Wiki (which I know isn't an entirely reputable source, but hey, it's not just me), it says "Yu Yevon had all the surviving citizens turn themselves into Fayth". Whether it is forced or not is never explicitly stated.

If maintaining Dream Zanarkand was the only reason for Sin's existence, then what about Sin's other thing, wiping out any large cities? "Yu Yevon gave this creature a set of very simple instructions: destroy any machina or settlement which grew larger than a small village in order to bring technical evolution to a halt, and retaliate to all hostility with extreme prejudice." - you be the judge of what Yu Yevon's intentions for these instructions were.

And Yunalesca began what is known as the teachings of Yevon because Yu Yevon was the one who is ultimately responsible for them, because he told her, and she passed it on. And since there was no source preceding Yu Yevon, he was the one who created the means of killing Sin...temporarily. So yes, Yunalesca was the main source for spreading the teachings and the religion, she didn't create them, Yu Yevon did. So of course she had part of it planned, she makes it quite clear that she likes the Spiral of Death when she tries to kill the party in Zanarkand.