Yes, but Diablo came out in 1997, when dial up was how you connected. Diablo 4 is releasing at a time when offline games are becoming a rarity.
And to be clear, the game is very much single-player. You do need to be online to play, and you’ll see other players run around, but almost all of the content is something you can do by yourself. There are raids where you need to be in a group, and you might want to team up with people for protection if you dabble in the PvP zones, but this isn’t an MMORPG that is expecting you to team up with a group to be able to do dungeons or anything. There are systems that encourage you to team up (such as experience boosts if another player is nearby) but very little is forcing you to do so.
You do need to be online to play, though. As in, you will always be connecting to a server, and if it’s undergoing maintenance, or you don’t have an internet connection, you aren’t going to play.
If you drop the cash for the game, you don’t have to pay another dime if you don’t want to. The money they’re asking for cosmetics is a bit crazy, where you could find yourself spending $25 to have a special suit of armor (note, only appearance, no stat boosts or abilities or anything). But that’s it. This isn’t Diablo Immortal, where your character’s power is tied strongly to “Resonance”, which mostly comes from high end gems, which almost never drop unless you spend real-life money to get special boosters that guarantee a gem will drop (but you only have a 1% chance of getting the gem you actually need, so you literally spend thousands of real life dollars to reach the top end of power in the game).
I will also say that when I played the beta, the customization was really nice. I don’t just mean during character creation (which was fine, for an ARPG it was pretty good actually), but I mean that as you find loot it unlocks appearances. You can then find a powerful ugly helmet or sword and change the appearance to something you like, and even customize colors (at least on armor) so you don’t look like a color-blind hobo on a quest to save the world. All of that is free and built in. You can unlock many really cool appearances just playing the game. The only reason to use a microtransaction is if you want to run around in some unique armor that has lightning flickering around the shoulders or whatever, and then feel special because other people didn’t buy that when it was available. (There is a bit of “fear of missing out” built in.) But this isn’t Path of Exile, where unless you spend money on a cosmetic set you’re forced to always look like you let your five-year-old niece dress you up from random clothes she found in her parents’ closet.
On the subject of expansion packs, reportedly they plan to release huge chunks of content (for free) every 3 months. New dungeons, new quests, new areas, new loot, and so on. It’s set up to encourage people to play indefinitely, just like an MMORPG (without actually being one, not really).