All current projects have fixed and defined milestones however the development of software and maintenence are not one-time effort. Following agile development, it's better to have checkpoints between milestones which gives fixed monthly grants to developpers. Also, due to the fluctuation of crypto coins, a monthly grant adjusted by the market price will more reliably compenstate developpers. The current and future grant amount should also be flexible or adjustable depending on future feature ask and requirements change.