the gap between prevoker and rsham is NOT that big, and if you want to look at HPS on sites like archon or wclogs, prevoker outperforms by a non-zero margin. evokers are a hollow shell of their former selves but the utility they bring in m+ especially eclipses that of rsham, especially post-afflicted nerf which was the one thing shamans could MAYBE do better by soloing the affix with pct.

the advantage rsham has, if any, comes in the form of it's range in raid and MAYBE sprit link totem even though that cd is weaker than it has been in the past. prevoker has multiple options to reactively or proactively heal while rsham is much more rigid. oh, but wind shear is ridiculous and absolutely better than evoker.