This would be remarkable for any game, but it’s particularly striking because WoW Classic is essentially the re-release of a 15-year-old title—complete with rough-edged character models and, arguably, outdated gameplay.

How many people are trying to play the game is unclear. But a commonly used yardstick in gaming — the number of people watching players broadcasting their experiences with a game on streaming services like Twitch—shows huge interest.

Currently, World of Warcraft (including both Classic and the current game) is the top overall game on streaming site Twitch, with around three times the viewers of more recent craze Fortnite. On Monday, the launch day for WoW Classic, the number of simultaneous viewers of World of Warcraft peaked at over 1.1 million, compared to the game’s recent average simultaneous viewership of around 50,000.

WoW Classic’s developer, Blizzard Entertainment—a unit of Activision Blizzard–is cautiously deploying more servers to accommodate demand. But the company has signaled it believes the current buzz is temporary.

Because WoW is a shared online world, players must log in to a server to play. On Wednesday afternoon, even with many players likely still at work, several servers showed wait times of two hours or more.

The dynamics behind the rollout of WoW Classic have been brewing since well before developer Blizzard announced the relaunch in 2017.

The original game, which premiered in 2004, was the biggest multiplayer online role-playing game of its time. By 2017, it had generated more than $9 billion in revenue for Activision Blizzard.

Over time, the game evolved. For example, Blizzard added regions and characters to the virtual world while changing some of the gameplay and other mechanics.

Criticism about the tweaks eventually pushed Blizzard to announce WoW Classic, which returns the game, more or less, to the original. The current ‘retail’ version of World of Warcraft will continue uninterrupted.

Although it could add new servers to fix the long wait time, Blizzard has said that it would instead move slowly. A community representative wrote in the game’s forums on Tuesday said that Blizzard is steadily adding new servers, but that it is waiting for each ‘realm’ to fill up before opening a new one.

That, according to the representative, is “because we want to make sure that each and every realm has a healthy population in the long term.” The implication is that Blizzard anticipates significant numbers of players to abandon the game after playing it for a short while.

There’s good reason for the concern. Though a vocal segment of players has long complained that Blizzard’s changes to the current version of World of Warcraft made it too easy, the contrasting challenge and time demands of WoW Classic may surprise players.

That challenge in WoW Classic includes a much slower process of ‘leveling’ to make characters more powerful, compared to the current version of the game. It’s also more difficult to form groups with other players to tackle tougher areas in the game.

So while WoW Classic’s launch is clearly a positive for Activision Blizzard, the risk is that players quickly become frustrated with it and therefore ditch it for something else. “[I] probably agree with management that WoW Classic is a marginal positive,” said Joe Bonner of Argus Research. “Something to fill in the gap until the big titles hit later in the fall.”

Activision Blizzard could use a new hit game or two. Subscriptions and other metrics for Hearthstone and Overwatch, its main games other than World of Warcraft, have declined in recent quarters.

That weakness has depressed Activision Blizzard’s shares. In November, they slumped from above $83 to the upper $40s, where they remain today. But if nostalgia keeps its hold and WoW Classic becomes a sleeper hit, the future may be brighter.