And how did you actually test this? Unless you physically disconnected your computer from the Internet or used a decent third-party firewall to block network access for steam.exe, then you're not actually offline - Steam can (and will) still make online checks, though less frequently, as noted in
Valve's Offline FAQ and confirmed by
multiple reports from other Steam users.
I don't need to test it. Two years ago, I had an internet outage from a storm. Gone w/o internet for 2 days. That's how I found out about Steam's offline mode. Every once in awhile, I'll also lose internet connection (checked the modem status) for a few hours and I had to use offline mode. With that said, I can't speak for future versions of steam or if steam will go under. Personally, even if 10 or 20 years from now that Steam changes their offline feature, it wouldn't really matter for me. Decade old games by then will be so cheap it wouldn't even be a concern to rebuy it all.
