I'm 31 and had an OATS (same as mosaicplasty) almost a year ago on my MFC. I have some extenuating circumstances (recurrent medial instablity, arthritis in other areas of the joint, AVN of the femoral condyle) that make me not the ideal candidate for OATS. Even with that, my OS's (two of them) think my OATS should last 5-10 years before TKR. So I'm not sure why the 3-5 years!
In my case, I could get TKR now. But at my age and with my activity level (I spend 80+ hours a week on my feet) my TKR wouldn't probably last 10 years. Even with my postop problems of AVN and fracturing my femoral condyle, the OATS has definitely been worth it.
As long as I'm gentle with the knee, it lets me work all day and rarely need any pain medication (I'm talking tylenol here). Before the OATS I had pain so bad I didn't sleep for 6 months. Even if I only get a few years out of it, it was well worth it. I've already had 9 surgeries on this knee and I know for sure that I will need a TKR in the future. Yes it's annoying, but the OATS has given me my life back and 5-10 years is a significant amount of time.