I'm a Boostrap lightweight, but I always respect an organization willing to drop support for old technology (IE8) and rewrite libraries with an improved technology (ES6).
Dropping old tech means projects/companies that require that support have to do it in-house. We need to support IE7+ for much of our UI, unfortunately, because of paying customers in hospital environments where upgrading is a nightmare. So it's a good thing in that libraries can be leaner/meaner for everyone who doesn't need to care about old tech, but it also means the unpleasant work of supporting the old tech now…
> When we shipped Bootstrap 3, we immediately discontinued all support for v2.x, causing a lot of pain for all our users out there. That was a mistake we won’t be making again. For the foreseeable future, we’ll be maintaining Bootstrap 3 with critical bug fixes and documentation improvements.
The current plan is already keeping basic support in place for BS3, but no new features. That's probably just where we'll stay, then.