It is highly unusual to have systemic reactions to an intrarticular steroid injection. As you were told, they tend to stay inside the joint capsule which is why/how they provide temporary relief. It is however temporary when there is joint surface breakdown. I did have one 1.5y ago. It gave me relief for a month or 2 but that's it. I had one in my thumb for a trigger. It worked perfectly. I'm happy to report that since about 8w after having it the thumb has not triggered once!
You say you have a really bad knee. There comes a point where NOT getting the replacement causes much more severe damage both in that leg and the other that is being overused to compensate. Have you considered that you've reached that breaking point and really need to consider the replacement?
I'm doing this dance myself. It's a balancing act of figuring out the right time of not too early and not too late! I have a large chunk missing from the thighbone toward the front. I cannot fully extend my leg without serious and severe pain. One bad move takes 3-4 days to recover from........