This is a great opporunity for startups - both getting funded on crowdfunded sites and launching crowdfunded sites, because unlike the usual ad-supported business model or saas offering, serious money changes hands and every sector of the economy can be flowing through this.
I doubt their fee will go down ever.
As regards ebay, their network effect hasn't worked out - amazon has taken their business. In the future Amazon or Google might come in take out all the crowdfunding sites by offering a reverse bounty api. At that point, the crowdfunding sites should move in to the function of providing marketing services for creators seeking funding.
1) Creating an account and entering banking numbers etc. is a real friction which is not easy to overcome.
2) People can go on Kickstarter and see a number of projects which they can fund without any prior knowledge of the person they are funding.
3) eBay is a $45B business which almost all of the value comes from network effects. Sure, Amazon is doing great (80B+) and is taking some of their business, but eBay is still a cash machine.
4) Looking at Indigogo it says that if you raise the money the fee is 4%. If you don't raise your goal the fee is 9%. While the 4% is better than 5%, my guess is that the total expected fee is actually above 5%.
6) Crowdfunding is not as simplistic as it appears on the surface. While not having the funding as an investment gets around many of the issues associated with the Securities and Exchange Commission, making a mistake could land an owner in jail or with substantial fines. Even if the owner did not mean to make something sound like an investment, if they do, it is a huge penalty. A simply API to create many more reverse crowd funded sites would not work out well on the average.