How many other JS build tools & frameworks from 2014 can say the same?

It's on Meteor 1.6 now, and you'd be mistaken about "much changes". Blaze to React was a nearly a rewrite; Atmosphere packages to npm was a significant change for the internal dependencies we developed; removing the custom file loader itself that preceded Atmosphere packages meant touching every single file.

The original code might have still worked, but the code would be helplessly outdated and stuck on a dying platform's technologies. As it is, though, the extensive and continuing effort to update the code means that now, we have a React/node app with an idiosyncratic webchannel connection to Mongo and a build process that's outdated but manageable, and still under active development of new features.

I credit the Meteor Development Group with recognizing that the stack they wanted to lock people in on, wasn't going to work (Blaze, Atmosphere, Mongo) and providing a paced transition off those pieces early on. But if we hadn't kept pace, we'd have a half million LOC app that would be termed "legacy".

We made the same transition, but just replacing one package/ui part at a time when it suited us, so we always kept releasing new features each sprint while gradually modernizing our code base.

Build tools wise, a simple Webpack/Babel config from 2014 would be relatively straightforward to update to the latest versions.