I don't think the effects of Cuprinol would last forever, but if air can get to the treated wood to dry it out after getting wet, it should be fine.
The major place to watch out for is the area under the door frames. water that gets in here is held in by the sill padding, and can't easily get away. In the old days MMC used to use horsehair padding, which did a great job of promoting rot in the wood.
So long as your car has a chance to dry out properly after getting wet, either in a nice airy car port or a dehumidified garage where damp can't build up, I would think the wood would never suffer from the dreaded rot. The key is air circulation. Anything that seals or holds damp into the wood is bad, anything that gets air circulating past it is good.
