Using one of those file hosting sites provides a greater level of
convenience. Perhaps so much greater that without that level of convenience
it would hardly be possible at all.
I'm surprised to hear that come from you. I've never used a cloud backup and the most I've ever lost is a day or two's worth of data/ media. I have redundant backups. It's not difficult (it truly isn't, I'm not trying to be snotty.)
The consumers don't care to invest in
security very much, in fact, hardly at all.
Do you mean the same lusers who broadcast the fact that they're on vacation all over Failbook, post photos with GPS enabled and are then surprised when their home is burglarized?
* full disk crypto is not a thing in androidland ;(
Sadly, it's not a "thing" anywhere right now. Not when EC has been intentionally weakened, etc. Hell, even if crApple did have true full disc encryption, I wouldn't use their closed source crapware.
tl;dr: javascript could be fine if we'd have secure software - as it is
HTML/CSS/images/videos/etc are all also dangerous. Top level security seems
(and often is) useless - therefore we don't really have it (even when we'd
like it so very much) unless we keep ourselves from essential features.
"Essential" is very much a subjective term. I don't mind most of my web browsing experience looking like plaintext (in fact, I much prefer it.) However, I understand most people do not want to use the web in that way. We all make concessions we consider acceptable, sacrificing privacy/security for convenience. I'm guilty of it, too. Anyone with a smartphone and a credit/debit card is as well.