Although the cost of cloud backups are egregious compared to the cost of raw storage, I'll pay a premium for the peace of mind that comes from letting someone else be held liable for storing my data and credentials to access it. I can easily upload/download files to google, iCloud, or Dropbox from almost any device knowing only my email and password, which I find preferable to having to remember an arbitrary 29 word seed. With the amount of exit scams in cryptocurrency, too, I just don't trust any project to continue to provide the same amount of utility that they do now, if they provide any at all.
I suppose you could use a custodian site to link an email and password to the seed, but then you enter a centralized third party to the mix.
I see value in Sia but it's just not for the average person in its current state.
You could store your Sia seed for free on Google Drive. That way you get the benefit of Google being "liable" for your data and credentials, but with the much lower storage costs of Sia. Yes, it's a centralized third party, but you already implied you have no problem with that.
Sia's seeds are ridiculous-- the 29 words provide 300 bits of entropy. 100 bits would be a sufficient security margin against brute forcing, assuming a modern memory-hard KDF like Argon2.
With a 100 bit password, assuming every flop of the 1.8 exaflops of the Top500 supercomputers tested a new password, it would still take 25,000 years to crack. Key stretching should add at least 30 bits of security by taking a billion operations--
Here's what 100 bits of security margin looks like with a more sophisticated scheme (abbrase): "Hope raised between unpleasant bellows. Devil rode sullenly, refugees waiting." => (first three letters) hopraibetunpbeldevrodsulrefwai.
I suppose you could use a custodian site to link an email and password to the seed, but then you enter a centralized third party to the mix.
I see value in Sia but it's just not for the average person in its current state.