> [...] it seems likely they also have enough to maliciously chug the hardware sufficiently to degrade capacity over time and otherwise impact system integrity. I hate the thought of some random website writing and overwriting random bytes in a tight loop in the background while I'm browsing elsewhere to find the cause of my slow disk subsystem.

Absolutely. Things like IndexedDB get fsynced super frequently. There's no way to tell Chrome that some web apps do not need to make it do the physical disk this often.