I am not a traffic engineer, but all way stops do need to meet federal warrants. There's more flexibility than with traffic signals though.
Couple of things. The RRFB beacon at the pedestrian crossing would need to be removed because the intersection would be under stop control. That may or may not work better for people crossing the street.
It's also possible that an all way stop would make Derby and Fairway busier, since it would be safer/easier to cross Waltham St. We see that at Eddy/Eliot now. The real and perceived danger of crossing Watertown St discouraged drivers before. Now more drivers use that route.
Conversely, Waltham St might be a little less appealing, and some traffic might end up on Cherry St.
Whether any of those things are good, bad, or acceptable depends on your view and possibly where you live, drive or walk. But it's worth thinking about all of them before contemplating a change.
-Mike
That doesn't argue