True -- if the service stays up. In practice, I've lost more data from online services shutting down than I ever did from old disks in my possession. Gmail and Facebook are probably going to be around forever, but the rest, who knows. The web itself isn't even 30 years old yet, so I don't feel confident predicting my data is going to survive there another 30.
When a disk format is obsolete, I can throw the disk in a shoebox and come back a couple decades later. When a web service shuts down, I have months/weeks/days (or maybe no chance at all) to get my data out, and then it's destroyed.
Yes, but disk drives (readers) are harder and harder to get.
I have 5.25" floppy disks with some awesome software I wrote at the university which is now probably lost (even if I had a drive the content is probably gone). Same for 3.5"s, zips,...
Unless you left them near something magnetic at some point, allowed them to mildew, or left them in a very hot car, they're quite possibly still readable. They are remarkably stable over time. See all of 4am's work for examples :-)
When hardware progress occurs, files are transferred to new hardware and you could expect that 30 years from now, your files will still be there.