This is probably not a convincing argument, but I'll share it anyway.
Think of bootstrapable builds as a hedge against tyranny.
Is it likely that a future regime could come into being that would be backdooring FLOSS builds and undermining confidence in the entire ecosystem? Not at all likely, as this requires a purge of all the install/boot media already out there.
Though after the passage of decades, the chain of custody starts to weaken. Maybe I'll still believe in the magic of the CD in my retro closet with the three humans holding hands in a circle, but in an environment where children are told to report the thoughtcrimes of their elders, how can my nephews and nieces in turn trust me and my testimony that my copy is the "good stuff" from "the before times".
This is all very far fetched sci-fi material, but think in terms of tail risk, low probability multiplied by a high cost if comes to pass.
Perhaps a regime horrible enough to break FLOSS would be horrible enough to purge all legacy hardware and software sitting in closets, but I do think it is easier to mess up the internet and the social fabric than it is to do house to house cleanouts.
Maybe messing around with untainted FLOSS will be too dangerous in such an environment anyway, but if we can assume a state of liberty eventually returns, having a great bootstrap ready to go can help rebuild society, cyberspace, and trust on an accelerated basis.
All of this hinges on people actually auditing source code, though once you've read enough bootstrapped source to have confidence in the generation of checksums you can at least start to fall back on a social consensus as to the safety of additional source archives that have known checksums and from there also digital signatures once you've built the tools for validating those.
When it comes to "trust, but verify" of other people's source code, there is a new tool in the toolbox: large language models have demonstrated some considerable audit value. Of course almost nobody is training their own models and it's only a slightly larger circle that does inference on their own hardware or attested remote hardware, but all of this is at least something that can increase confidence when working from other folks source code.
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As far fetched as full on cyber tyranny sounds, its worth remembering that in North Korea this is actually reality. Folks do not have access to the internet and are handed the state built binary of Red Star OS.
The ballooning of flash drives containing western media is still a thing despite attempts to stop it.
It is unlikely that anyone will ever send in a software bootstrap, that it will get in the hands of someone who can understand it, have the time to read it and build it.
But the scenario does make us think about the value of bootstrapable FLOSS as a tyranny hedge.
Without real internet access, what good does having some freedom on a computer even do? I suppose one can keep a diary encrypted, though possessing an encrypted file is probably more of a liability than any value it provides. Someone may also find encryption helpful for friend to friend networks by sneakernet, though the downsides are considerable.
Someway, somehow the regime may fall. After decades of not trusting the outside world and being governed by a hostile state, perhaps there will be at least one deeply computer literate person to emerge from that place who will appreciate being able to bootstrap their own personal cyber citadel.