I had a house in Phila. with a tile roof, built in 1920. I "replaced" the roof in 1995, and by that I literally mean re-placed: the roofers carefully took all the tile off the roof, put down two layers of 80lb roofing felt, and reinstalled all the tile. They believed that the original was done with only 1 layer of 40lb roofing felt, and that this was the only thing that had actually failed other than a couple of cracked tiles. There was some expectation that the new one might last 100 years before it would need the same process again.

What was the failure mode of your tile roof that meant it needed replacing? Coming from the UK where most roofs are tile I don’t think I ever saw one need replacing there, maybe a few tiles need realigning. Most tile roofs were the same as the house itself, so over a century for large swaths of the country. The weather is less extreme than in the US though.

"Tile" in the US probably (though not definitively) means "spanish tile" - those terracotta curved tiles. It's not the same as what I remember tile meaning in the UK, which was typically slate.

Also, I spoke with some roofers in Pennsylvania who explained to me the difference between Welsh slate and the stuff from PA ... way less durable because the layers are not as tightly bonded, and then you add freeze/thaw and it just doesn't last as long.

correct, Also the underlay needs to be replaced often and if it's not the tiles can come loose or get damaged. Replacing the underlay requires removing all the tile, which is labor intensive, and then depending on the HOA/owner preferences, you can try to color match damaged tiles or replace the tiles, or replace portions, all of these are done regularly. Depending on type of underlay used, it might need to be replaced every 7-25 years, again depending on the weather conditions and materials used.