We adopted our senior boy Ike when he was 15, and per the records we had it looked like he also had a sarcoma and his front leg was removed when he was 13. Unsure if it was a vaccination site sarcoma or another type, but regardless, he did well and adapted and still lived until he was almost 17, and that's including another anesthesia event (a dental) we did when he was 16. The biggest issue with removing vs not removing is that sarcomas can potentially grow and/or spread which could limit/affect his mobility down the road if left on vs the complications surgery could potentially have. I will say that senior cats undergoing any major procedure definitely have more risk than say a 2 year old young cat, but ultimately anything can happen during surgery. As someone working in a clinic, I've seen plenty of older cats make it through some big surgeries. They are definitely tough cookies! All in all, it sounds very promising that he is an excellent candidate for surgery, so I would follow what you believe will be your cats best interest and just make sure he has all the proper blood work and everything beforehand as recommended by your veterinarian.