Which "classic" Blizzard games are we talking about here? The entire Diablo franchise since Diablo 2 has had a battle.net element associated with it. Diablo was ruined constantly by cheaters, and Diablo 2 went a long way to stopping the most obvious character editor cheats if you chose to play on one of their online realms. The downside being that if the realm was down then you couldn't play that character, and that if you chose to play offline then you were siloed off in your own little world where you would never encounter anyone else except maybe friends on your local LAN (or you could use a service like Kali lol).
Yeah it's nice to have offline mode until you realize that all your offline progress can't be mirrored online where your friends or your clan play.
People act like Diablo was so much better for its offline play, since you could pop online with any character you wanted, but the consequence was that you had to deal with other people doing the same thing (with cheated characters), plus the awful townkill hacks and invulnerable players etc.
You can't reasonably expect all of those issues to be 100% solved without issue.
With Diablo 2 Blizzard offered two Battle.net options: Closed and Open Battle.net.
Closed Battle.net was online 100% of the time, characters stored on server, can only be played on games running on battle.net. This was the most secure from hacking and bugs, but like all online systems was not invulnerable.
Open Battle.net, where you can run local file singleplayer characters, play singleplayer, LAN, or login to Battle.net and create open Battle.net games and play with other characters.
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On open Battle.net, by design, you can't control what people's character files contain because they're simply locally saved on their computers and vulnerable to file editors.
I'm not really sure what kind of solution you have in mind that allows fully safe, unmodifiable character files but also allows offline and online play without any of these vulnerabilities?
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On another hand, if you agree that online stored characters are the most secure way to play without hacking, cheating, and modification, what are you really complaining about with regards to server outages or maintenance? It's a fact of life that is very difficult to work around.
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Pretty sure the legitimate complaint with Diablo 3 (and now 4) is you are not allowed to play offline at all in any capacity. if I don't want to play online with other players, I have no choice. If I have no internet connection, I cannot play.
I feel like you're conflating the reasons for offline available characters vs. online only characters, with forced online only play.