Well, it's not a doomsday argument to say that there's something wrong with fiat. Neither is it a doomsday scenario to describe fiat as perpetually losing value.
Of course inflation exists and fiat indeed tends to lose value slowly (this is also politically intended, due to the problems associated with deflation by the classical economics schools -- maybe wrongly), but the fiat system provides a lot of means to preserve your value. In most years if you hold government bonds for example, then you will be able to counter inflation successfully. Not always (2022/23 was a prime example) but mostly. The "Doomsdayers" instead predict hyperinflation or constant high inflation since ... ancient times? Well at least since the 90s, and probably since the end of the Bretton Woods system.
But in the rest of the post it seems you got what I meant with the "Doomsdayers", so I wrote that only to clarify
A sort of doomsday scenario?
It's a difference if you (mentally) prepare for a worst case scenario with a probability of let's say 5% in 30-40 years, or if you sell entire books predicting the end of the world in at most 5 years
What's good with Bitcoin, one which assures me 100%, is that the narrative doesn't matter, so far at least. Whether it's falsely associated-- and it was-- with drugs, pedophiles, gamblers, black market, clueless geeks, traditional financial giants, politicians, political parties, ideologies, and so on and so forth, at the end of the day, everybody can verify themselves that the technology is indeed decentralized, neutral, censorship-resistant, immaculate.
Regarding technology you're correct, but I disagree a bit that the narrative doesn't matter at all. A "popular and universally accepted Bitcoin" is probably a much better scenario as a "niche Bitcoin with support of a few Central Banks", even if both could lead to the same Bitcoin price. Above all, the "popular Bitcoin" scenario would be more sustainable, while the "Bitcoin for a few" or "Bitcoin for special people" (like another forum member has called it earlier in this thread), inherently, can lead to overly restrictive regulations (justified with "the majority population doesn't like Bitcoin") which counter Bitcoin's advantages and makes it more difficult to benefit from them.
The widespread KYC obligations for exchanges, for example, a product of the "Bitcoin is for criminals" narrative, make it more difficult for opposition movements in dictatorships to raise funds with Bitcoin (and Monero, etc.).