Long-range attacks are an easy problem to solve definitively with very little cost. All blocks with six months to one year of history accumulated on top of it should be socially declared final, historical fact. In fact, socially-finalized transaction histories should be replaced by social-consensus snapshots. These save space and help speed up synchronization. Periodic snapshotting should be the job of the “Foundation.” It is impossible to cheat on that, so it doesn’t matter if there’s only one entity providing that service (others can do it as well, but it would be just a waste of time).
To leave it up to the consensus mechanism running on full nodes to not revert ancient history is an unnecessary risk, regardless of the block-production mechanism. It is just better to institute periodic snapshotting as a social/community organization mechanism.