Peter, I would bet a likely reason your friends turned over their DACs more than TT setups is because digital is improving faster (and has more room to improve) versus analog. It may have nothing to do with satisfaction if that's what you are implying.
I agree that digital has more room to improve. We have read many reports though, and recently, that digital is extremely satisfying and yet, the search continues for something ever more satisfying.
My earlier comment was really in response to the cost thing. Someone wrote that good digital is half the cost of good analog or something. Well, if I keep my analog for a long time, and another guy keeps changing his DAC, those changes can add up. My point being, that I wonder if people actually save much money in the longer term if they keep changing DACs compared to the vinyl alternative. It is an open question for me.
Some of the digital companies are responding to the technology changing and offer modular designs. I think dCS and MSB do this. That might be less expensive in the long term than constantly changing DACs from different companies searching for improvements or change. I don't know the answer to that.
And yes, some guys buy cartridges every year or so and those can cost more than many DACs. As I wrote earlier, it's a personal thing.