removing fiber IS a breaking change for every application, in particular if you are using pub-sub with meteor collections.
Fibers was primarly used to make this isomorphic:
this returns the results synchronous on both client and server (there is no await). On client its done by Tracker running the surrounding function whenever the data change and on the server its done with fibers.
Removing fibers would require to use await here and/or to give up isomorphic code.
This is a very challenging problem. maintaining fiber code is probably the easier way out. And deprecating the whole meteor.Collections data layer and replace with something new…