...what's to stop governments or the FED/ ECB to start own digital tokens (decentralized or not) and declare them national currency? It would be absolutely in their interest, get rid of cash and even gain control over the money flow!
They already have it, it's called fiat currency and mostly exist in digital form.
Even in this case Bitcoin will still be the real deal, safe and expensive, used by big players and a security for your retirement, but you won't go buy groceries with it. You won't see millions of transactions, neither now nor in the future. Yet, 1 Bitcoin will cost 3000 Dollars or more. It will be a huge success, just a bit different from you thought it would be.
Sure, Bitcoin is like gold, except gold was around for thousands of years to establish itself as something valuable and there's still no guarantee that people one day won't turn away from it. Theres also demand for gold (jewelery, electronic components) which adds to the value. With no merchant adoption and practical usage Bitcoin will likely fade away.
Having Bitcoin only good as an investment and other, emerging cryptocurrency which can be used for both investment and day-to-day transaction - which one will win in the long run?
I don't get where you guys are coming from. If you take away any practical usage, you essentially fully rely on unidentified 'big players' who will just keep throwing more money in Bitcoin for some reasons. But most of all you would make Bitcoin fully controlled by governments and banks, since it could only be used on (regulated) exchanges.