This was such a good answer by Tall_Allen, I thought it deserved its own post.
Most people have the wrong model of metastatic prostate cancer in their heads. They think of it as a weed that crops up: Keep after the weeds long enough and eventually no more weeds.
A more accurate metaphor is mushrooms growing at the base of an oak tree. The mycelium extends everywhere throughout the soil and into the roots of the tree. Occasionally, a mushroom crops up. You can pick all the mushrooms you want, but the fungus is never destroyed. There is no way to destroy the fungus short of destroying the roots of the oak tree and sterilizing the soil. This is what "systemic" means.
Can systemic treatment someday POTENTIALLY cure it? Maybe. Cancer, like HIV, hides in reservoirs. It is very hard to get to all the places it may be hiding. It evolves rapidly - any therapy-resistant cells eventually come to dominate. That means that there will always be some cells that don't rely on androgen-receptor activation, some that don't express PSA or PSMA, and some that aren't killed by microtubule stabilization (docetaxel). It also changes the microenvironment in the places where it nests so that they become more fertile for invasion. I'm talking about "cure" rather than "control."
I think that if we are ever going to control it, it will be through multimodal therapies (as for HIV) that attack it from several directions at once. So far, growth pathway inhibitors have been disappointing (the sheer number of growth pathways is daunting and increases as the cancer mutates), as have Immunotherapies - PC does not engender a big immune response and mimics "self" cells. Perhaps, a combination will be more effective - or maybe CAR-T. There is always hope.
Real nice post with a lot more detail on this subject: