I agree with OP, getting all that money early decreases a dev's desire/need to push through and finish remaining content.
If you were offered $1000 to do a job, but you only get the money once the job is done, you would work a lot harder than if it was the same job, but you got $400 of the $1000 at the start.
I agree, but the reason the crowd funding is looked at is that the project would not have gone ahead without it. so it is a bit of a loose / loose situation. All the more that people should be told they are investing in something that might not pay back. In fact, it might be better if the project only offered discount vouchures for the end product and not actual copies. There will be less people "buying in", but each will have a better understanding of what is going on.
As to the second part, true, but if that job takes too long or the person gets a better offer to go and do something else, why keep working for wishful thinking. In real life terms, getting told you will get a pay rise if you work hard, then two years later still having not seen that pay rise, you feel cheated and go out of yourway to find something else if not only stick around and work half has hard as you use to.
With the idea of keeping the money from the person doing the job, it raises other issues like what to do with the money as the backers and the project can not have the money. It has to be somewhere (collected, ready to pay) but you have to trust where that money is placed. Banks can go belly up, and individuals could steal it (ie: the ones looking after the money). it does happen. Then eveyone looses.
The project needs the money otherwise they would not be asking for it, so holding it all back is pointless. Not like they can go and get funding using the raised money as a bond. It just shifts the risk to a third party.
Another option is like the banks and building a house (at least in my country) where milestones need to be met before the money for the next stage can be released. It might work but it needs definied milestones and who is going to be able to say the project has reached them? And if they do not , what happens to the remaining money? finding all the backers to pay them back $5 is unlikly given someone will charge administration charges onto finding the person to give it back, so the invester is still out of money.