Serms don't lower estrogen. They simply block the receptors preventing estrogen from acting on the body. When in PCT your natural testosterone is going to be crashed. However, your body will still have a lot of excess estrogen floating around in your system creating an estrogen dominant hormone imbalance. This can lead to (gyno, bloating, ED, acne, losing all your gains) basically everything you don't want after a cycle. Not only that, serms also boost LH significantly. Helping you get back to normal levels faster. (ie: a month or 2 instead of 6 months).
as for joint lubrication. You shouldn't have problems. Now with an AI such as aromasin or letro which actually "kill" estrogen. You will experience low estro symptoms if you dose it high enough. Estrogen isn't a bad thing. The key is keeping it under control which can be tricky sometimes.