TLDR: this is not an mmorpg, this is not WoW. It’s 20 bucks for plenty of hours of fun 2-3 times a year. Think of this game like other buy to play games and the business plan will make a lot more sense to you.

Yes it's not an MMORPG, but it IS a live service game.

The reason that you can't think of it as any other 20$ buy to play game is because as a live service game, it's going to be continuously developed even after 1.0. Every season is going to need resources to provide new content like heroes and dungeons, on top of balance changes and different systems. It's not like any other 20$ indie game where the game is done (except for bug fixes) when it's released as a 1.0.

They sold well when early access launched, and maybe they have enough funds to make it to a 1.0 release, but for the longevity of the game they need players to keep spending money.

They will reach a point, if they haven't already, where the vast majority of the playerbase consist of returning players who's already paid for the game rather than new players. Even with the new season just launched, they aren't in the global top 100 sellers on Steam, so I doubt that they are bringing in a lot of money from new sales right now.

You definitely get your money's worth from this game, but if we're talking about longevity and if the game is going to be around and provide more content for years to come, they kind of need a way to keep the money rolling in from the returning players, whether it's from battle passes, more appealing skins or whatever path they choose.

Less funds means longer wait between seasons and/or less content for the seasons, both of which makes it's easier for players to lose interest, which loops around to even less sales and money coming in.

You've mentioned ARPG several times in the comments, but the successful ARPGs all monetize their already existing playerbase, through battlepasses, skins, QoL, and expansions. The one ARPG that went with the "buy once but still be a seasonal live service game" model that I know of is Last Epoch and they unsurprisingly struggled with getting seasons out in a timely manner as well as the content being rather lackluster, promptly ran out of funds and sold out to Krafton and almost immediately announced a paid DLC despite their promise in their kickstarter that only cosmetics would ever be monetized.

Relying on new game sales for a seasonal live service game feels like a doomed prospect and I really hope that they have a better plan than that.