I'm struggling to get much info on the CSS process, but according to Thorn brochures the tungsten carbide fuses with the metal on the rim. So that would suggest you wouldn't be getting absolutely the FULL normal rim wear once the top surface has worn away. But all evidence so far points to VERY long rim life.
Does anyone have any knowledge of the non-CSS life-expectancy of the most heavy-duty rims (Sun Rhyno, etc) ? It'd be interesting for anyone setting out on a very long ride to have an idea of whether they'd benefit long-term from CSS rims or not.
Approx distances of some of the more 'common' long distance rides would be:
UK - Cape Town: 20,000-25,000kms
Alaska - Patagonia: somewhere in the region of 25,000kms
RTW: Obviously massively variable, but anything but the most direct Russian-North American route is likely to be 30,000kms+
My (unconfirmed) suspicion- based on my own experience of moderately tough rims- is that the toughest non-CSS rims would get you through the first two of the above, but for multi-year trips (or a lifetime of medium-length touring) CSS would be the way to go.
I'd like to point out to anyone new to touring (and in danger of getting scared away by the silly prices of the gear being talked about!) that you don't NEED to spend hundreds of pounds on wheels to get into touring. Particularly on the front, which is under much less stress. But for a seriously long ride- particularly through developing countries- a high quality wheel will probably eventually save you money, and certainly peace of mind.