If an instrument is priced correctly, that is to say at the risk adjusted net present value of its future cash flows, then it absolutely will go up in the long

If an instrument is priced correctly, that is to say at the risk adjusted net present value of its future cash flows, then it absolutely will go up in the long term as it returns cash (dividends, buybacks) and as cash flows that are still in the future come closer to the present so become more certain and more valuable (time value). There is of course the question whether the instrument is currently priced correctly. That's why you diversify, ideally into uncorrelated investments so that misvaluations balance out on the upside and the downside.