If bitcoin becomes the global reserve, nothing will happen to gold. Because gold is not only an investment asset but also an important material in most industries and the demand for gold will not be affected just because of a digital asset like bitcoin.
Also, honestly, I even think the day bitcoin becomes a global reserve is still a long way off, I don't see it happening in this decade or the next. Look at what's happening now, how central banks around the world are treating bitcoin. They don't need it like we think, the FED has publicly stated that they have no intention of adding bitcoin to the federal reserve, or the ECB has even outright criticized bitcoin and considers it only a speculative asset, and not suitable to be a currency or reserve asset.
So let's forget about bitcoin replace gold as the global reserve anytime soon.
Not only that, but gold will not become a non-strategic thing, it would stay as a reserve too. If bitcoin becomes a global reserve, which it won't, because even gold is not as much as it used to be, that doesn't change the fact that nations will have gold too, they would just have bitcoin along with their gold.
But yeah, you are right, gold is already used in many things, it does have an instinct value if you think about it, even the computers that we use has small golds in them, tiny amounts of course but they are there. Hence why garbage companies exist, of which, they just gather as many garbage as they can, and they use machinery to get the valuable stuff out of them, and most of them do get some good returns for it.