90% of the time the answer to that question should just simply be - "feature creep." Some ideas might be worth incorporating and putting the time in, but only a select few should make it through this filter. "Feature creep" is said by engineers to draw a line in the sand and protect an incubating product from being pulled in too many directions at once. While it can seem dick-ish at times, this is an important front for the engineering team to hold because if they fail to, it can dramatically draw out the product development process and jeopardize the success of the project.
So, finish what you start, only pivot when necessary, and always be wary of "feature creep," because creepers gonna creep.