Ummm... sure... OK...
You could purchase a DX11 card as soon as they come out and yes, it will work in DX11 for it's entire life. However, no card's hardware performance on it's release is going to be on-par with newer, top-performing cards that come 6 months from then, let alone a year later and beyond. Therefore, no video card, nor any computer hardware at all, is ever "future-proof." Eventually, there comes a time where all video cards cannot perform at acceptable speeds, and the length of time seems to be growing shorter and shorter with how quickly computer technology has been advancing over recent years. It was the hardware to which I was referring, not the DirectX version.
Did the elongated version of what I said at first explain it any better?