There's no solution that will give PoS the same objectivity PoW enjoys, but PoS (with slashing and exit delays) is secure against long forks under a weakly subjective model.
Essentially, the solution is to ignore forks whose lengths exceed the exit delay, since such forks can be created without a slashing risk.
The main issue is that new nodes need to start with a (somewhat recent) trusted checkpoint. This does open up some attack surface that doesn't exist in PoW, though in my view it's not much of an issue. If I want to be extra careful, I can always ask several sources for a checkpoint and make sure they match.
Vitalik has a pretty good post about it: https://blog.ethereum.org/2014/11/25/proof-stake-learned-lov...
Essentially, the solution is to ignore forks whose lengths exceed the exit delay, since such forks can be created without a slashing risk.
The main issue is that new nodes need to start with a (somewhat recent) trusted checkpoint. This does open up some attack surface that doesn't exist in PoW, though in my view it's not much of an issue. If I want to be extra careful, I can always ask several sources for a checkpoint and make sure they match.