i too went with an aluminum seat very similar to the ultrashield pro road race.

my reasoning was as follows:

1. with aluminum i can see or easily test (dye penetrant) for damage that would be unseen on others.
2. with aluminum, i don't have to worry about a sanctioning body saying they have degraded due to UV.
3. "upgrading" to not have to bother with a seat back brace is nonsense imho.

the UV thing may never happen but many other things that were good "forever" until showing signs of being worn are now being replaced every 2-5 years. i know of no reason that kevlar may not show the same potential for damage and require periodic replacement.

google "UV damage kevlar" and you will find many hits
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I have yet to see a kevlar seat that was not fully painted like a fiberglass seat. UV damage is essentially zero.