Leaving aside GHCJS (which IMO is an entirely orthogonal issue), Haskell ecosystem is largely stuck with GHC 8.10, because there is no Stackage LTS for GHC 9.0. And there is no LTS, because GHC 9.0.1 was hardly usable. With a recent release of 9.0.2 and Stackage preparing for LTS 19, this difficulty is likely to be resolved pretty soon.
I strongly suspect that funnelling community efforts into faster migration of packages to newer GHCs has better ROI than embarking on decoupling base from GHC (which just delays the key issue). And if we can make GHC to evolve at a pace of a mature compiler and not an academic PoC, it would bring even higher ROI overall.
I entirely agree this decoupling base from GHC is a laudable enterprise, my doubts are just about potential ROI in comparison to other avenues we can choose to pursue (taking into account limited resources of community / HF / GHC developers).