I just can't see it NOT being a FIAT vacuum cleaner.
Don't worry, I don't think Bitcoin will ever be a "fiat vacum cleaner". Think of it this way....
There are 2 general types of "money" needed by advanced economies. 1 type that keeps prices stable (for trading) and one type that keeps the supply fixed (for store of value). The one that keeps prices stable is simply a way to denominate trades - it isn't actually "money" in the sense of being a bearer token like commodities, precious metals or cryptocurrencies. On many websites now you can even choose how to denominate your trade - Canadian Dollars, $USD, EUR, whatever.
Those trading currencies need to have a variable supply - like the USD. A fractional reserve system which inflates and contracts according to liquidity requirements. If you don't have that and use a fixed supply currency like Bitcoin for trading then half the businesses in the world will go bust and the other half will make supernormal profits which are not sustainable.
So I don't see bitcoin ever being used as a trading currency en masse. (You don't see prices ever being denominated in gold and neither did we when there was a gold standard). Bitcoin is a base asset which can be used to "back" trading currencies for example or facilitate the capitalisation of whatever markets, but I think prices will still continue to be denominated in prevailing regional currencies.
I should be clear as many have made the "vacuum cleaner" statement. What I am saying is that instead of having a savings account we will (at least) have "Bitcoin accounts". Sure, the money supply by governments will want to be contracted and expanded for reasons that both make sense and are sick. No argument that that will continue for a while, perhaps with different FIAT money though.
But me as a "working"

individual, I do NOT want to have my money sitting in government money as I don't trust it and don't like how it is being used and using us. I would much rather have it sitting in Bitcoin or something stable. Maybe we keep a few thousand in dollars for spending in a checking account (but maybe we don't as if BTC is accepted everywhere, why have anything in government money?)
I just think the numbers won't lie and that people, even if they are only losing a very small amount, say .1% of actual buying power per month, would not want to have their money in the regular bank and would much rather have it in a Bitcoin bank (under their immediate control or not) as it might sound better to them to gain .1% per month, rather than lose .1% per month.
Its about sharing
You make great and valid points, however most people don't have money sitting in the form of fiat. In fact, most people don't have a (relevantly) postive balance on their credits and debts at all! And those who do usually have them tied up in other things.
For example, being a (slightly above) average joe, I own a house. I am actually happy that the mortgage on that is denominated in fiat, because that devalues relative to the value of my house.
Fiat money usually gets spent very quickly. With interest rates being what they are, it's just no use keeping it.