While Bootstrap v3 is largely stable, this is an interesting development given the effort needed to maintain a (soon-to-be) legacy version measured against the resources needed to work on the future version.

Will people hop onto v4 alpha and boost the development effort there? Or will they stand by stable v3?

Will people transition to other frameworks in the interim?

Will somebody fork/maintain v3?

Why is this a necessity in bootstrap, where other large projects have lumbered on without taking such an action?

Have other large projects succeeded with a similar strategy? Or will other large projects follow suit?