Public blockchains[0] are not known to scale either. I can open a Twitter account for free and publish a signature right now, and do it on several other platforms at the same time to have some kind of redundancy.

I only care about the medium being tamper-proof to be able to prove the signature is at least this old (if it's in a certain block, the signature was made before this block. If it's in a certain message on Twitter, it was made before this message).

So from first principles, blockchains brings this theoretically better time-stamping mechanism, because somebody controlling Twitter could change timestamps there, while nobody could on a blockchain. In practice though, the redundancy is enough, and it's hard to change something people care about on the Internet without people noticing.

Overall this use-case somewhat legit (more legit than most), but it's a niche within a niche.

[0]: as defined in https://www.schneier.com/essays/archives/2019/02/theres_no_g...