What if what they have so far isn't fun, but you think it might become very fun if they were given enough money to finish it? Isn't that (sometimes) worth taking the risk on?
To steal an idea from Bungie, when it comes to games you want "thirty seconds of fun" -- a base gameplay loop (in the case of Halo, where it originally applied, it's the loop of taking out baddies from a distance, then the stronger baddies from mid-range, then mopping up the scattered weak baddies). An early demo (like the aforementioned Rust Legacy) is a proof-of-concept of that base gameplay loop, and if players like it, they can fund it.
Once it's funded, they can build out from that base loop, and eventually you'll get a finished product.
