Nothing, you make a good point. We were going to use github commits as a trusted oracle in our first prototype of this idea so that a supervisor would not be needed (i.e. you would pledge to commit to a certain project a given amount within the month) but this would still be easy to game in the way you described.
I had been discussing almost exactly this idea with a friend over the last couple weeks, wholly unaware of the similar services listed elsewhere in the comments (although I realise there is a chance I heard about one of them in the past but forgot about it). Good job on getting this up and running, looks great so far!
The way we had proposed to deal with funds that were not returned to the user because of failing a goal was to add them to a reward pool of sorts. We would take a percentage of any failed goal funds, but the majority would be given as rewards to others who had completed their goals in a similar area or time period. The details of this hadn't been fleshed out but one potential way to do it we discussed would be to distribute the reward pool at the end of every month to those who had completed gals in that month (proportional to the amount of money staked).
This would potentially remove some people's concerns listed in the comments about the money not being handled well in case of goal failure. Also it might drive more usage of the service if people would maybe even profit.
This is incorrect
XMR is the token name of Monero
ADA is the token name of Cardano