This is also what I used to think, bitcoin will become an asset like gold and will be the choice in the budget reserves of countries. But even if that happens, it will never be able to replace fiat, the government will never let that happen. Given the speed of the world, it is certain that the fiat system will be upgraded, but it will still be a centralized and government-run system. There can't be a decentralized asset like bitcoin that can replace existing fiat.
Governments always want control/power over the mainstream economy, so they will reject Bitcoin in every way possible. Of course, that doesn't mean they won't add BTC to their balance sheet like they've been doing with Gold a long time ago. What it means is that governments won't let BTC replace Fiat for their own convenience. This shouldn't be a reason of concern, especially when BTC is decentralized. You can't replace centralization with decentralization or vice-versa.
I'm fine with Bitcoin just the way it is, as long as people achieve true financial freedom. In the future, BTC will live alongside existing Fiat currencies and even precious metals for generations. People will get to decide which currency/asset to use depending on their needs. Who knows if almost everyone in the world gets to "own" some Bitcoin in the future? Just my thoughts