Seems unlikely to me that using a clutch is the primary cause of your knee problem. I have always driven a stick until the last few years, so about the same amount of time as you before that, but my knee problem is due to an ACL injury over 30 years ago. I’m sure using a clutch had some minor contribution to all of the cartilage wearing away and the meniscus falling apart but it was gonna happen anyway. My right knee isn’t fine either, all that time it’s been carrying the extra load for my left knee has worn it out. Might need that replaced next year. Remember that part and keep your right knee in good shape while your left has problems.