Well, the central bank thing is just the latest guidelines from the Bank for International Settlements, issued before there were ETFs and before states started to stealth their way into BTC mining and hoarding.

If they find that BTC is "too volatile" to the upside, they will simply amend the guideline to allow for a higher %, or to give it a boom value only equivalent to the one they bought at or whatever other trickery they come up with, at their convenience.

And as far as downside goes, same thing, but I think the fact that central banks themselves hold BTC will be an anchor to eliminate a lot of the volatility *once we reach a large enough market cap*.