Hi, one of the authors of Backpack here.
Why has Backpack not taken off? The biggest reason is no one is working full time on it. Since finishing my PhD, I’m being paid to work on a different open source project (PyTorch), and between that and raising a new baby, I don’t really have time beyond helping answer people’s questions about Backpack in threads like this 
But let’s say that we did have someone pushing Backpack (something I’d love to see! I still believe in the project). What are the things they’d have to do first? Here’s my ordered list:
- Implement Stack support for Backpack (this is the number one reason why regular libraries can’t easily go off and start using Backpack, unless it’s entirely internal use it locks you out of the Stackage ecosystem)
- Introduce Backpack support to GHC’s build system (as the most important libraries you might want to parametrize are base which are built as part of GHC’s bootstrap process)
- Fix some of the major outstanding Backpack bugs; specifically https://github.com/haskell/cabal/issues/6835 is the most important one, https://github.com/haskell/cabal/issues/5434 is not too bad but pretty annoying
These are pretty hefty projects. But if you want to replace String in base, I don’t really see any other route!