Yevonism also had a grip upon the school system. They were the ones that taught children and what better way to indoctrinate someone than to get them while they are young. America has been doing it for years, as has Japan, Palestinians etc and so forth. Even if they were just teaching the children how to read, write and do arithmetic, some indoctrination of the religion is going to seep through and teach the children their morals (revere your elders and ancestors, don’t murder, don’t steal, it’s all right for Summoners to die in the fight against Sin…) Yevon also did marriages. They were there, cradle to the grave. Plus, they gained new converts through the Guado and the Ronso. There had to be something there for entire cultures to shift over to it en masse.

And by no means did Yevonism die out after Yuna destroyed Sin, in fact that was a major part of the problem. They got a new name (not a very creative new name), new leaders and a new creed, but deep down they were still the same Yevon. It’s hard for a coeurl to change its stripes just by adding New to the title. The new goal of New Yevon was to help those who were scared and frightened after the defeat of Sin and the discovery of the fact of how corrupt Yevon had been. Deep down, they still had the same goal, control of Spira, which was why the Youth League was formed. New Yevon refused to share the history collected by sphere hunters and (spoiler alert) if you make your way to the bottom of Via Infinito and talk to Trema (the Leader before Baralai) you’ll discover that he has actually destroyed the history spheres because he believes the only way Spira can go into the future is to erase the past! He decided to do it quickly rather than over hundreds of years. (end of spoiler)

You say that the temples were deserted and abandoned. You’re thinking of Djose, which is rather off the beaten path (and I think we had some map cut there too). I wouldn’t be surprised if people stopped coming there. The Guado were in Macalania woods and the Humans were rushing to get to the Moonflow. Of the other temples that were deserted, most were already empty in the first place, Baaj (in the middle of the ocean), Remiem (hidden, cut off by an avalanche), Zanarkand (now a tourist attraction), Cavern of the Stolen Fayth (also a tourist attraction) and Macalania (fallen into the lake, such a pity). The other temples were still in use, which was another major problem. Besaid had a Priest on duty and people still visited, as it was right in the middle of ‘town’. Killika Temple was a refuge for the New Yevonites who couldn’t bear to leave in a Youth League dominated town and Bevelle was the headquarters for New Yevon and needed to be protected because their stewardship of Vegnagun was not complete. And Djose technically wasn’t abandoned. It just had new owners that’s all, Al Bhed being the opportunistic people that they are. Because there were people in these temples and around these temple it was important to go in and fight the ‘heretical’ aeons controlling the fiends so people weren’t hurt. If these temples had been abandoned then this wouldn’t matter! The fiends could have kept pouring out of them until the encroached on cities and towns and must be destroyed and people wouldn’t even know until they went looking for the source of the problem!

You seem very focused on one aspect of the game. There was a lot more out there that was developed than 120 hours you play through.

I hope I made my points a little clearer and didn’t come off as a prig.