And like lachek pointed, they will never release a game with infinite replay value.
Qualification: they will never release a game with infinite replay value without charging recurring fees for it. Which, I imagine, is also why games supporting easy community modding are few and far between, despite the incredibly passionate and talented communities that build up around games which
do support it (Civ IV, Neverwinter Nights (number one), etc). To be commercially successful in today's landscape, a one-time-buy game should be based on a well-known middleware game engine; have 15-20 hours scripted gameplay, expandable by DLCs; a shallow multiplayer component to cater to the FPS crowd; and tons of marketing. A subscription-based game should have a vast game world of repetitive "content", Skinner-box-style game mechanics, and be driven by multiplayer components which keep the content and mechanics fresh with minimal development effort.
Dwarf Fortress is none of these - it's a never-ending gold mine of continuously fresh gameplay, with no multiplayer and no scripted events. You're creating a personal product while playing the game, an artefact which is intensely
yours. This means you can't charge recurring fees for it, and you can't force its obsolescence so people go out and buy the latest and greatest iteration after they're "done" with an earlier edition. I suppose you could charge "upgrade fees" for improved editions - like one would with e.g. MS Office - or a subscription-for-updates like Mount & Blade or Minecraft - but given the amount of sheer gameplay available already, that seems like a tricky customer proposition, esp. without a marketing department.
No, the current model fits best, as long as it keeps Tarn fed and housed: it's an alpha and will remain so for years, you can download and play it if you want, if you like it and want to support its development, donate. Without getting too haughty or pretentious, I see it as a piece of art - it's something Tarn does for himself first and foremost, but he's okay with other people looking at it while he's making it, and he's using that as a promotional vehicle to solicit commissions from the connoisseurs who want to see him continue to improve upon it.
No true artist ever went into their profession because they wanted to crank out stuff the market liked and make gobs of cash. First and foremost they're in it for their own appreciation of what they're producing, and the degree they need to engage in commercialism is based on what they absolutely have to charge to continue doing what they do. For those of us firmly entrenched in a market economy, with priorities like hoarding more stuff for our clan (or its euphemism "taking care of the family"), this can easily appear as deviant, even selfish behaviour, when in fact it's quite the opposite.