If they solve the anonymity problem (such that they can replace paper currency, which can also be entirely anonymous), then I'd expect that you could buy anything you want with it.
Subject to anti-money-laundering rules, of course, which usually apply to larger transactions.
> it won't triple in value in one year
That doesn't have much to do with the currency, but with the market, and volatility works both ways.
...and 15bn EUR banknotes. If you think about it, they all have a serial number and it's easy to track them with a mere bigdata database. Most banknotes change hands only once before being registered in a bank again. The others are all suspicious. Keeping your money for a few months? Not a crime by itself but in most situations it's linked with laundering. Withdrawing every week from your ATM and cashed by the same drug dealer? They probably won't scream it over the roofs, but the FBI probably already uses this technique to track criminals.
If you think it's hard to track patterns in banknotes, it's ok, but it's less hard than tracking every single CB transaction across the world.
> As a risk-free, interest-bearing asset, CBcoin would be preferable to bank deposits (and even paper currency, presuming anonymity concerns were addressed), encouraging households and firms to convert their bank deposits into CBcoin deposits.
While governments might not prefer anonymous payment methods to exist, they can still recognize that much of the money-using public appreciates anonymous payments at least some of the time.
To the extent that "governments" don't like anonymous payment methods, that's mostly law enforcement/security agencies.
Central banks basically recognize that anonymous payments mechanisms will exist, and prefer that central bank currency fill that role, since the more non-central-bank money is used in practice, the less predictable effect central bank monetary policy has.