Good lord has it been that long?

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The original code might have still worked, but the code would be helplessly outdated and stuck on a dying platform's technologies.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that now, we have a React/node app with an idiosyncratic webchannel connection to Mongo and a build process that's outdated but manageable, and still under active development of new features.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

But if you had picked any other tool from 2014, you'd probably also be stuck with a legacy code base now, without any clear transition path except a big bang rewrite. You would have probably picked angularjs or backbone back in 2014 and a switch to react would have been even more trouble. Meteor allows you to mix blaze & react, mix atmosphere & npm packages, mix their old module systems with commonjs and ES6 modules.

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.

True: we'd have faced the same transition with any other stack. In hindsight, Meteor wasn't a bad choice, it just didn't live up to its promise of sidestepping churn and offering an opinionated stack that would endure. I don't know that any did.

Not exactly a framework, but a React app from 2014 will still work great in 2019 and can be updated with very little pain.

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

How many JS build tools from 2014 exist? And how many are still used? I think both will be fairly close to zero.

That's my point. Other people had to rewrite their software for new build tools, while our Meteor app started 5 years ago never needed any big changes, while we could gradually adopt ES6+ features, modules, dynamic imports, ...

Are Grunt and Gulp still not used?

*aren't -not

Maybe in really old and ossified projects?