Well no shit radiation takes forever to go away in real life, but that's not how it works at all in the Fallout universe. It's based off the physics and such of 1950's Science and World of Tomorrow! stuff. The mod's altering of how radiation stays around longer than a few days to months aside from certain areas basically pisses on the lore of the setting, much like how Bethesda implied that a Mr. Handy unit actually uses oil for fuel instead of a miniature atomic generator. The way radiation works in the Fallout setting is the only justifiable reason the games even take place in the manner in which they do. Shady Sands wouldn't even fucking exist in Fallout 1 because the Vault Dweller entered Shady Sands on December 15th, 2161, just over 84 years after the bombs dropped. In real life, that place would be worse than Chernobyl, but it's not set in real life, hence why DUST and FROST's usage and interpretation of radiation won't sit well with people who try to use mods that adhere to the lore.