How long a rim lasts is one of those how long is a piece of string questions. It depends on so many factors it’s impossible to say with any degree of certainty. I’ve worn out a Mavic touring rim in 7,000 miles of gravely bike path commuting and had a lighter road rim last 25,000 + miles. So although 12,000 doesn’t sound great, it’s not the worst. I think you just have to treat modern rims as consumables, especially lightweight ones. Find yourself a good local wheelbuilder, the hub and spokes will last several rims. I prefer rims that have a wear indicator, so I know when to replace them before they break. The CR18 is my present 26” favorite.